GENERAL CONSIDERATIONS ON THE MOVEMENTS AND GROWTH OF SEEDLING PLANTS.
Generality of the circumnutating movement—Radicles, their
circumnutation of service—Manner in which they penetrate the
ground—Manner in which hypocotyls and other organs break through the
ground by being arched—Singular manner of germination in Megarrhiza,
etc.—Abortion of cotyledons—Circumnutation of hypocotyls and epicotyls
whilst still buried and arched—Their power of straightening
themselves—Bursting of the seed-coats—Inherited effect of the arching
process in hypogean hypocotyls—Circumnutation of hypocotyls and
epicotyls when erect—Circumnutation of cotyledons—Pulvini or joints of
cotyledons, duration of their activity, rudimentary in Oxalis
corniculata, their development—Sensitiveness of cotyledons to light and
consequent disturbance of their periodic movements—Sensitiveness of
cotyledons to contact.
The circumnutating movements of the several parts or organs of a
considerable number of seedling plants have been described in the last
chapter. A list is here appended of the Families, Cohorts, Sub-classes,
etc., to which they belong, arranged and numbered according to the
classification adopted by Hooker.[1] Any one who will consider this
list will see that the young plants selected for observation, fairly
represent the whole vegetable series excepting the lowest cryptogams,
and the movements of some of the latter when mature will hereafter be
described. As all the seedlings which were observed, including
Conifers, Cycads and Ferns, which belong to the most ancient
types amongst plants, were continually circumnutating, we may infer
that this kind of movement is common to every seedling species.
[1] As given in the ‘General System of Botany,’ by Le Maout and
Decaisne, 1873.
SUB-KINGDOM I.—Phaenogamous Plants.
Class I.—DICOTYLEDONS.
Sub-class I.—Angiosperms. Family. Cohort. 14. Cruciferae. II.
PARIETALES. 26. Caryophylleae. IV. CARYOPHYLLALES. 36. Malvaceae. VI
MALVALES. 41. Oxalideae. VII. GERANIALES. 49. Tropaeoleae. DITTO 52.
Aurantiaceae. DITTO 70. Hippocastaneae. X. SAPINDALES. 75. Leguminosae.
XI. ROSALES. 106. Cucurbitaceæ. XII. PASSIFLORALES. 109. Cacteæ. XIV.
FICOIDALES. 122. Compositæ. XVII. ASTRALES. 135. Primulaceae. XX.
PRIMULALES. 145. Asclepiadeae. XXII. GENTIANALES. 151. Convolvulaceae.
XXIII. POLEMONIALES. 154. Boragineae. DITTO 156. Nolaneae. DITTO 157.
Solaneae. XXIV. SOLANALES. 181. Chenopodieae. XXVII. CHENOPODIALES.
202. Euphorbiaceae. XXXII. EUPHORBIALES. 211. Cupuliferae. XXXVI.
QUERNALES. 212. Corylaceae. DITTO
Sub-class II.—Gymnosperms. 223. Coniferæ. 224. Cycadeæ.
Class II.—MONOCOTYLEDONS. 2. Cannaceae. II. AMOMALES. 34. Liliaceae.
XI. LILIALES. 41. Asparageae. DITTO 55. Gramineæ. XV. GLUMALES.
SUB-KINGDOM II.—Cryptogamic Plants.
1. Filices. I. FILICALES. 6. Lycopodiaceæ. DITTO
Radicles.—In all the germinating seeds observed by us, the first change
is the protrusion of the radicle, which immediately bends downwards and
endeavours to penetrate the ground. In order to effect this, it is
almost necessary that the seed should be pressed down so as to offer
some resistance, unless indeed the soil is extremely loose; for
otherwise the seed is lifted up, instead of the radicle penetrating the
surface. But seeds often get covered by earth thrown up by burrowing
quadrupeds or scratching birds, by the castings of earth-worms, by
heaps of excrement, the decaying branches of trees, etc., and will thus
be pressed down; and they must often fall into cracks when the ground
is dry, or into holes. Even with seeds lying on the bare surface, the
first developed root-hairs, by becoming attached to stones or other
objects on the surface, are able to hold down the upper part of the
radicle, whilst the tip penetrates the ground. Sachs has shown[2] how
well and closely root-hairs adapt themselves by growth to the most
irregular particles in the soil, and become firmly attached to them.
This attachment seems to be effected by the softening or liquefaction
of the outer surface of the wall of the hair and its subsequent
consolidation, as will be on some future occasion more fully described.
This intimate union plays an important part, according to Sachs, in the
absorption of water and of the inorganic matter dissolved in it. The
mechanical aid afforded by the root-hairs in penetrating the ground is
probably only a secondary service.
[2] ‘Physiologie Végétale,’ 1868, pp. 199, 205.
The tip of the radicle, as soon as it protrudes from the seed-coats,
begins to circumnutate, and the whole
growing part continues to do so, probably for as long as growth
continues. This movement of the radicle has been described in Brassica,
Æsculus, Phaseolus, Vicia, Cucurbita, Quercus and Zea. The probability
of its occurrence was inferred by Sachs,[3] from radicles placed
vertically upwards being acted on by geotropism (which we likewise
found to be the case), for if they had remained absolutely
perpendicular, the attraction of gravity could not have caused them to
bend to any one side. Circumnutation was observed in the above
specified cases, either by means of extremely fine filaments of glass
affixed to the radicles in the manner previously described, or by their
being allowed to grow downwards over inclined smoked glass-plates, on
which they left their tracks. In the latter cases the serpentine course
(see Figs. 19, 21, 27, 41) showed unequivocally that the apex had
continually moved from side to side. This lateral movement was small in
extent, being in the case of Phaseolus at most about 1 mm. from a
medial line to both sides. But there was also movement in a vertical
plane at right angles to the inclined glass-plates. This was shown by
the tracks often being alternately a little broader and narrower, due
to the radicles having alternately pressed with greater and less force
on the plates. Occasionally little bridges of soot were left across the
tracks, showing that the apex had at these spots been lifted up. This
latter fact was especially apt to occur
xwhen the radicle instead of travelling straight down the glass made a
semicircular bend; but Fig. 52 shows that this may occur when the track
is rectilinear. The apex by thus rising, was in one instance able to
surmount a bristle cemented across an inclined glass-plate; but slips
of wood only 1/40 of an inch in thickness always caused the radicles to
bend rectangularly to one side, so that the apex did not rise to this
small height in opposition to geotropism.
[3] ‘Ueber das Wachsthum der Wurzeln: Arbeiten des bot. Instituts in
Würzburg,’ Heft iii. 1873, p. 460. This memoir, besides its intrinsic
and great interest, deserves to be studied as a model of careful
investigation, and we shall have occasion to refer to it repeatedly.
Dr. Frank had previously remarked (‘Beiträge zur Pflanzenphysiologie,
1868, p. 81) on the fact of radicles placed vertically upwards being
acted on by geotropism, and he explained it by the supposition that
their growth was not equal on all sides.
In those cases in which radicles with attached filaments were placed so
as to stand up almost vertically, they curved downwards through the
action of geotropism, circumnutating at the same time, and their
courses were consequently zigzag. Sometimes, however, they made great
circular sweeps, the lines being likewise zigzag.
Radicles closely surrounded by earth, even when this is thoroughly
soaked and softened, may perhaps be quite prevented from
circumnutating. Yet we should remember that the circumnutating
sheath-like cotyledons of Phalaris, the hypocotyls of Solanum, and the
epicotyls of Asparagus formed round themselves little circular cracks
or furrows in a superficial layer of damp argillaceous sand. They were
also able, as well as the hypocotyls of Brassica, to form straight
furrows in damp sand, whilst circumnutating and bending towards a
lateral light. In a future chapter it will be shown that the rocking or
circumnutating movement of the flower-heads of Trifolium subterraneum
aids them in burying themselves. It is therefore probable that the
circumnutation of the tip of the radicle aids it slightly in
penetrating the ground; and it may be observed in several of the
previously given diagrams, that the movement is more strongly
pronounced in radicles when they first
protrude from the seed than at a rather later period; but whether this
is an accidental or an adaptive coincidence we do not pretend to
decide. Nevertheless, when young radicles of _Phaseolus multiflorus_
were fixed vertically close over damp sand, in the expectation that as
soon as they reached it they would form circular furrows, this did not
occur,—a fact which may be accounted for, as we believe, by the furrow
being filled up as soon as formed by the rapid increase of thickness in
the apex of the radicle. Whether or not a radicle, when surrounded by
softened earth, is aided in forming a passage for itself by
circumnutating, this movement can hardly fail to be of high importance,
by guiding the radicle along a line of least resistance, as will be
seen in the next chapter when we treat of the sensibility of the tip to
contact. If, however, a radicle in its downward growth breaks obliquely
into any crevice, or a hole left by a decayed root, or one made by the
larva of an insect, and more especially by worms, the circumnutating
movement of the tip will materially aid it in following such open
passage; and we have observed that roots commonly run down the old
burrows of worms.[4]
[4] See, also, Prof. Hensen’s statements (‘Zeitschrift für Wissen,
Zool.,’ B. xxviii. p. 354, 1877) to the same effect. He goes so far as
to believe that roots are able to penetrate the ground to a great
depth only by means of the burrows made by worms.
When a radicle is placed in a horizontal or inclined position, the
terminal growing part, as is well known, bends down towards the centre
of the earth; and Sachs[5] has shown that whilst thus bending, the
growth of the lower surface is greatly retarded, whilst that
of the upper surface continues at the normal rate, or may be even
somewhat increased. He has further shown by attaching a thread, running
over a pulley, to a horizontal radicle of large size, namely that of
the common bean, that it was able to pull up a weight of only one
gramme, or 15.4 grains. We may therefore conclude that geotropism does
not give a radicle force sufficient to penetrate the ground, but merely
tells it (if such an expression may be used) which course to pursue.
Before we knew of Sachs’ more precise observations we covered a flat
surface of damp sand with the thinnest tin-foil which we could procure
(.02 to .03 mm., or .00012 to .00079 of an inch in thickness), and
placed a radicle close above, in such a position that it grew almost
perpendicularly downwards. When the apex came into contact with the
polished level surface it turned at right angles and glided over it
without leaving any impression; yet the tin-foil was so flexible, that
a little stick of soft wood, pointed to the same degree as the end of
the radicle and gently loaded with a weight of only a quarter of an
ounce (120 grains) plainly indented the tin-foil.
[5] ‘Arbeiten des bot. Inst. Würzburg,’ vol. i. 1873, p. 461. See also
p. 397 for the length of the growing part, and p. 451 on the force of
geotropism.
Radicles are able to penetrate the ground by the force due to their
longitudinal and transverse growth; the seeds themselves being held
down by the weight of the superincumbent soil. In the case of the bean
the apex, protected by the root-cap, is sharp, and the growing part,
from 8 to 10 mm. in length, is much more rigid, as Sachs has proved,
than the part immediately above, which has ceased to increase in
length. We endeavoured to ascertain the downward pressure of the
growing part, by placing germinating beans between two small metal
plates, the upper one of which was loaded with a known weight; and the
radicle was then allowed to grow into a narrow hole in wood, 2 or 3
tenths of an inch in depth, and closed at the bottom. The wood was so
cut that the short space of radicle between the mouth of the hole and
the bean could not bend laterally on three sides; but it was impossible
to protect the fourth side, close to the bean. Consequently, as long as
the radicle continued to increase in length and remained straight, the
weighted bean would be lifted up after the tip had reached the bottom
of the shallow hole. Beans thus arranged, surrounded by damp sand,
lifted up a quarter of a pound in 24 h. after the tip of the radicle
had entered the hole. With a greater weight the radicles themselves
always became bent on the one unguarded side; but this probably would
not have occurred if they had been closely surrounded on all sides by
compact earth. There was, however, a possible, but not probable, source
of error in these trials, for it was not ascertained whether the beans
themselves go on swelling for several days after they have germinated,
and after having been treated in the manner in which ours had been;
namely, being first left for 24 h. in water, then allowed to germinate
in very damp air, afterwards placed over the hole and almost surrounded
by damp sand in a closed box.
Fig. 55. Outline of piece of stick (reduced to one-half natural size)
with a hole through which the radicle of a bean grew. Thickness of
stick at narrow end .08 inch, at broad end .16; depth of hole .1 inch.
We succeeded better in ascertaining the force exerted transversely by
these radicles. Two were so placed as to penetrate small holes made in
little sticks, one of which was cut into the shape here exactly copied
(Fig. 55). The short end of the stick beyond the hole was purposely
split, but not the opposite
end. As the wood was highly elastic, the split or fissure closed
immediately after being made. After six days the stick and bean were
dug out of the damp sand, and the radicle was found to be much enlarged
above and beneath the hole. The fissure which was at first quite
closed, was now open to a width of 4 mm.; as soon as the radicle was
extracted, it immediately closed to a width of 2 mm. The stick was then
suspended horizontally by a fine wire passing through the hole lately
filled by the radicle, and a little saucer was suspended beneath to
receive the weights; and it required 8 lbs. 8 ozs. to open the fissure
to the width of 4 mm.—that is, the width before the root was extracted.
But the part of the radicle (only .1 of an inch in length) which was
embedded in the hole, probably exerted a greater transverse strain even
than 8 lbs. 8 ozs., for it had split the solid wood for a length of
rather more than a quarter of an inch (exactly .275 inch), and this
fissure is shown in Fig. 55. A second stick was tried in the same
manner with almost exactly the same result.
Fig. 56. Wooden pincers, kept closed by a spiral brass spring, with a
hole (.14 inch in diameter and .6 inch in depth) bored through the
narrow closed part, through which a radicle of a bean was allowed to
grow. Temp. 50°–60° F.
We then followed a better plan. Holes were bored near the narrow end of
two wooden clips or pincers (Fig. 56), kept closed by brass spiral
springs. Two radicles in damp sand were allowed to grow through these
holes. The
pincers rested on glass-plates to lessen the friction from the sand.
The holes were a little larger (viz..14 inch) and considerably deeper
(viz..6 inch) than in the trials with the sticks; so that a greater
length of a rather thicker radicle exerted a transverse strain. After
13 days they were taken up. The distance of two dots (see the figure)
on the longer ends of the pincers was now carefully measured; the
radicles were then extracted from the holes, and the pincers of course
closed. They were then suspended horizontally in the same manner as
were the bits of sticks, and a weight of 1500 grams (or 3 pounds 4
ounces) was necessary with one of the pincers to open them to the same
extent as had been effected by the transverse growth of the radicle. As
soon as this radicle had slightly opened the pincers, it had grown into
a flattened form and had escaped a little beyond the hole; its diameter
in one direction being 4.2 mm., and at rightangles 3.5 mm. If this
escape and flattening could have been prevented, the radicle would
probably have exerted a greater strain than the 3 pounds 4 ounces. With
the other pincers the radicle escaped still further out of the hole;
and the weight required to open them to the same extent as had been
effected by the radicle, was only 600 grams.
With these facts before us, there seems little difficulty in
understanding how a radicle penetrates the ground. The apex is pointed
and is protected by the root-cap; the terminal growing part is rigid,
and increases in length with a force equal, as far as our observations
can be trusted, to the pressure of at least a quarter of a pound,
probably with a much greater force when prevented from bending to any
side by the surrounding earth. Whilst thus increasing in length it
increases in thickness, pushing away the damp
earth on all sides, with a force of above 8 pounds in one case, of 3
pounds in another case. It was impossible to decide whether the actual
apex exerts, relatively to its diameter, the same transverse strain as
the parts a little higher up; but there seems no reason to doubt that
this would be the case. The growing part therefore does not act like a
nail when hammered into a board, but more like a wedge of wood, which
whilst slowly driven into a crevice continually expands at the same
time by the absorption of water; and a wedge thus acting will split
even a mass of rock.
Manner in which Hypocotyls, Epicotyls, etc., rise up and break through
the ground.—After the radicle has penetrated the ground and fixed the
seed, the hypocotyls of all the dicotyledonous seedlings observed by
us, which lift their cotyledons above the surface, break through the
ground in the form of an arch. When the cotyledons are hypogean, that
is, remain buried in the soil, the hypocotyl is hardly developed, and
the epicotyl or plumule rises in like manner as an arch through the
ground. In all, or at least in most of such cases, the downwardly bent
apex remains for a time enclosed within the seed-coats. With Corylus
avellena the cotyledons are hypogean, and the epicotyl is arched; but
in the particular case described in the last chapter its apex had been
injured, and it grew laterally through the soil like a root; and in
consequence of this it had emitted two secondary shoots, which likewise
broke through the ground as arches.
Cyclamen does not produce any distinct stem, and only a single
cotyledon appears at first;[6] its petiole
breaks through the ground as an arch (Fig. 57). Abronia has only a
single fully developed cotyledon, but in this case it is the hypocotyl
which first emerges and is arched. Abronia umbellata, however, presents
this peculiarity, that the enfolded blade of the one developed
cotyledon (with the enclosed endosperm) whilst still beneath the
surface has its apex upturned and parallel to the descending leg of the
arched hypocotyl; but it is dragged out of the ground by the continued
growth of the hypocotyl, with the apex pointing downward. With Cycas
pectinata the cotyledons are hypogean, and a true leaf first breaks
through the ground with its petiole forming an arch.
[6] This is the conclusion arrived at by Dr. H. Gressner (‘Bot.
Zeitung,’ 1874, p. 837), who maintains that what has been considered
by other botanists as the first true leaf is really the second
cotyledon, which is greatly delayed in its development.
Fig. 57. Cyclamen Persicum: seedling, figure enlarged: c, blade of
cotyledon, not yet expanded, with arched petiole beginning to
straighten itself; h, hypocotyl developed into a corm; r, secondary
radicles.
Fig. 58. Acanthus mollis: seedling with the hypogean cotyledon on the
near side removed and the radicles cut off; a, blade of first leaf
beginning to expand, with petiole still partially arched; b, second and
opposite leaf, as yet very imperfectly developed; c, hypogean cotyledon
on the opposite side.
In the genus Acanthus the cotyledons are likewise hypogean. In A.
mollis, a single leaf first breaks through the ground with its petiole
arched, and with the opposite leaf much less developed, short,
straight, of a yellowish colour, and with the petiole at first not half
as thick as that of the other. The undeveloped leaf is protected by
standing beneath its arched fellow; and it is an
instrucive fact that it is not arched, as it has not to force for
itself a passage through the ground. In the accompanying sketch (Fig.
58) the petiole of the first leaf has already partially straightened
itself, and the blade is beginning to unfold. The small second leaf
ultimately grows to an equal size with the first, but this process is
effected at very different rates in different individuals: in one
instance the second leaf did not appear fully above the ground until
six weeks after the first leaf. As the leaves in the whole family of
the Acanthaceae stand either opposite one another or in whorls, and as
these are of equal size, the great inequality between the first two
leaves is a singular fact. We can see how this inequality of
development and the arching of the petiole could have been gradually
acquired, if they were beneficial to the seedlings by favouring their
emergence; for with A. candelabrum, spinosus, and latifolius there was
a great variability in the inequality between the two first leaves and
in the arching of their petioles. In one seedling of A. candelabrum the
first leaf was arched and nine times as long as the second, which
latter consisted of a mere little, yellowish-white, straight, hairy
style. In other seedlings the difference in length between the two
leaves was as 3 to 2, or as 4 to 3, or as only .76 to .62 inch. In
these latter cases the first and taller leaf was not properly arched.
Lastly, in another seedling there was not the least difference in size
between the two first leaves, and both of them had their petioles
straight; their laminae were enfolded and pressed against each other,
forming a lance or wedge, by which means they had broken through the
ground. Therefore in different individuals of this same species of
Acanthus the first pair of leaves breaks through the ground by two
widely different methods; and if
either had proved decidedly advantageous or disadvantageous, one of
them no doubt would soon have prevailed.
Asa Gray has described[7] the peculiar manner of germination of three
widely different plants, in which the hypocotyl is hardly at all
developed. These were therefore observed by us in relation to our
present subject.
[7] ‘Botanical Text-Book,’ 1879, p. 22.
Delphinium nudicaule.—The elongated petioles of the two cotyledons are
confluent (as are sometimes their blades at the base), and they break
through the ground as an arch. They thus resemble in a most deceptive
manner a hypocotyl. At first they are solid, but after a time become
tubular; and the basal part beneath the ground is enlarged into a
hollow chamber, within which the young leaves are developed without any
prominent plumule. Externally root-hairs are formed on the confluent
petioles, either a little above, or on a level with, the plumule. The
first leaf at an early period of its growth and whilst within the
chamber is quite straight, but the petiole soon becomes arched; and the
swelling of this part (and probably of the blade) splits open one side
of the chamber, and the leaf then emerges. The slit was found in one
case to be 3.2 mm. in length, and it is seated on the line of
confluence of the two petioles. The leaf when it first escapes from the
chamber is buried beneath the ground, and now an upper part of the
petiole near the blade becomes arched in the usual manner. The second
leaf comes out of the slit either straight or somewhat arched, but
afterwards the upper part of the petiole,—certainly in some, and we
believe in all cases,—arches itself whilst forcing a passage through
the soil.
Megarrhiza Californica.—The cotyledons of this Gourd never free
themselves from the seed-coats and are hypogean. Their petioles are
completely confluent, forming a tube which terminates downwards in a
little solid point, consisting of a minute radicle and hypocotyl, with
the likewise minute plumule enclosed within the base of the tube. This
structure was well exhibited in an abnormal specimen, in which one of
the two cotyledons failed to produce a petiole, whilst the other
produced one consisting of an open semicylinder ending in a sharp
point, formed of the parts just described. As soon as the confluent
petioles protrude from the seed they bend down, as they are strongly
geotropic, and penetrate the ground. The seed itself retains its
original position, either on the surface or buried at some depth, as
the case may be. If, however, the point of the confluent petioles meets
with some obstacle in the soil, as appears to have occurred with the
seedlings described and figured by Asa Gray,[8] the cotyledons are
lifted up above the ground. The petioles are clothed with root-hairs
like those on a true radicle, and they likewise resemble radicles in
becoming brown when immersed in a solution of permanganate of
potassium. Our seeds were subjected to a high temperature, and in the
course of three or four days the petioles penetrated the soil
perpendicularly to a depth of from 2 to 2½ inches; and not until then
did the true radicle begin to grow. In one specimen which was closely
observed, the petioles in 7 days after their first protrusion attained
a length of 2½ inches, and the radicle by this time had also become
well developed. The plumule, still enclosed within the tube, was now
.3 inch in length, and was quite straight; but from having increased in
thickness it had just begun to split open the lower part of the
petioles on one side, along the line of their confluence. By the
following morning the upper part of the plumule had arched itself into
a right angle, and the convex side or elbow had thus been forced out
through the slit. Here then the arching of the plumule plays the same
part as in the case of the petioles of the Delphinium. As the plumule
continued to grow, the tip became more arched, and in the course of six
days it emerged through the 2½ inches of superincumbent soil, still
retaining its arched form. After reaching the surface it straightened
itself in the usual manner. In the accompanying figure (Fig. 58, A) we
have a sketch of a seedling in this advanced state of development; the
surface of the ground being represented by the line G...........G.
[8] ‘American Journal of Science,’ vol. xiv. 1877, p. 21.
Fig. 58, A. Megarrhiza Californica: sketch of seedling, copied from Asa
Gray, reduced to one-half scale: c, cotyledons within seed-coats; p,
the two confluent petioles; h and r, hypocotyl and radicle; p1,
plumule; G..........G, surface of soil.
The germination of the seeds in their native Californian home proceeds
in a rather different manner, as we infer from an interesting letter
from Mr. Rattan, sent to us by Prof. Asa Gray. The petioles protrude
from the seeds soon after the autumnal rains, and penetrate the ground,
generally in a vertical direction, to a depth of from 4 to even 6
inches. they were found in this state by Mr. Rattan during the
Christmas vacation, with the
plumules still enclosed within the tubes; and he remarks that if the
plumules had been at once developed and had reached the surface (as
occurred with our seeds which were exposed to a high temperature), they
would surely have been killed by the frost. As it is, they lie dormant
at some depth beneath the surface, and are thus protected from the
cold; and the root-hairs on the petioles would supply them with
sufficient moisture. We shall hereafter see that many seedlings are
protected from frost, but by a widely different process, namely, by
being drawn beneath the surface by the contraction of their radicles.
We may, however, believe that the extraordinary manner of germination
of Megarrhiza has another and secondary advantage. The radicle begins
in a few weeks to enlarge into a little tuber, which then abounds with
starch and is only slightly bitter. It would therefore be very liable
to be devoured by animals, were it not protected by being buried whilst
young and tender, at a depth of some inches beneath the surface.
Ultimately it grows to a huge size.
Ipomœa leptophylla.—In most of the species of this genus the hypocotyl
is well developed, and breaks through the ground as an arch. But the
seeds of the present species in germinating behave like those of
Megarrhiza, excepting that the elongated petioles of the cotyledons are
not confluent. After they have protruded from the seed, they are united
at their lower ends with the undeveloped hypocotyl and undeveloped
radicle, which together form a point only about .1 inch in length. They
are at first highly geotropic, and penetrate the ground to a depth of
rather above half an inch. The radicle then begins to grow. On four
occasions after the petioles had grown for a short distance vertically
downwards, they
were placed in a horizontal position in damp air in the dark, and in
the course of 4 hours they again became curved vertically downwards,
having passed through 90° in this time. But their sensitiveness to
geotropism lasts for only 2 or 3 days; and the terminal part alone, for
a length of between .2 and .4 inch, is thus sensitive. Although the
petioles of our specimens did not penetrate the ground to a greater
depth than about ½ inch, yet they continued for some time to grow
rapidly, and finally attained the great length of about 3 inches. The
upper part is apogeotropic, and therefore grows vertically upwards,
excepting a short portion close to the blades, which at an early period
bends downwards and becomes arched, and thus breaks through the ground.
Afterwards this portion straightens itself, and the cotyledons then
free themselves from the seed-coats. Thus we here have in different
parts of the same organ widely different kinds of movement and of
sensitiveness; for the basal part is geotropic, the upper part
apogeotropic, and a portion near the blades temporarily and
spontaneously arches itself. The plumule is not developed for some
little time; and as it rises between the bases of the parallel and
closely approximate petioles of the cotyledons, which in breaking
through the ground have formed an almost open passage, it does not
require to be arched and is consequently always straight. Whether the
plumule remains buried and dormant for a time in its native country,
and is thus protected from the cold of winter, we do not know. The
radicle, like that of the Megarrhiza, grows into a tuber-like mass,
which ultimately attains a great size. So it is with Ipomœa pandurata,
the germination of which, as Asa Gray informs us, resembles that of I.
leptophylla.
The following case is interesting in connection with
the root-like nature of the petioles. The radicle of a seedling was cut
off, as it was completely decayed, and the two now separated cotyledons
were planted. They emitted roots from their bases, and continued green
and healthy for two months. The blades of both then withered, and on
removing the earth the bases of the petioles (instead of the radicle)
were found enlarged into little tubers. Whether these would have had
the power of producing two independent plants in the following summer,
we do not know.
In Quercus virens, according to Dr. Engelmann,[9] both the cotyledons
and their petioles are confluent. The latter grow to a length “of an
inch or even more;” and, if we understand rightly, penetrate the
ground, so that they must be geotropic. The nutriment within the
cotyledons is then quickly transferred to the hypocotyl or radicle,
which thus becomes developed into a fusiform tuber. The fact of tubers
being formed by the foregoing three widely distinct plants, makes us
believe that their protection from animals at an early age and whilst
tender, is one at least of the advantages gained by the remarkable
elongation of the petioles of the cotyledons, together with their power
of penetrating the ground like roots under the guidance of geotropism.
[9] ‘Transact. St. Louis Acad. Science,’ vol. iv. p. 190.
The following cases may be here given, as they bear on our present
subject, though not relating to seedlings. The flower-stem of the
parasitic Lathraea squamaria, which is destitute of true leaves, breaks
through the ground as an arch;[10] so does the
flower-stem of the parasitic and leafless Monotropa hypopitys. With
Helleborus niger, the flower-stems, which rise up independently of the
leaves, likewise break through the ground as arches. This is also the
case with the greatly elongated flower-stems, as well as with the
petioles of Epimedium pinnatum. So it is with the petioles of
Ranunculus ficaria, when they have to break through the ground, but
when they arise from the summit of the bulb above ground, they are from
the first quite straight; and this is a fact which deserves notice. The
rachis of the bracken fern (Pteris aquilina), and of some, probably
many, other ferns, likewise rises above ground under the form of an
arch. No doubt other analogous instances could be found by careful
search. In all ordinary cases of bulbs, rhizomes,
root-stocks, etc., buried beneath the ground, the surface is broken by
a cone formed by the young imbricated leaves, the combined growth of
which gives them force sufficient for the purpose.
[10] The passage of the flower-stem of the Lathraea through the ground
cannot fail to be greatly facilitated by the extraordinary quantity of
water secreted at this period of the year by the subterranean
scale-like leaves; not that there is any reason to suppose that the
secretion is a special adaptation for this purpose: it probably
follows from the great quantity of sap absorbed in the early spring by
the parasitic roots. After a long period without any rain, the earth
had become light-coloured and very dry, but it was dark-coloured and
damp, even in parts quite wet, for a distance of at least six inches
all round each flower-stem. The water is secreted by glands (described
by Cohn, ‘Bericht. Bot. Sect. der Schlesischen Gesell.,’ 1876, p. 113)
which line the longitudinal channels running through each scale-like
leaf. A large plant was dug up, washed so as to remove the earth, left
for some time to drain, and then placed in the evening on a dry
glass-plate, covered with a bell-glass, and by next morning it had
secreted a large pool of water. The plate was wiped dry, and in the
course of the succeeding 7 or 8 hours another little pool was
secreted, and after 16 additional hours several large drops. A smaller
plant was washed and placed in a large jar, which was left inclined
for an hour, by which time no more water drained off. The jar was then
placed upright and closed: after 23 hours two drachms of water were
collected from the bottom, and a little more after 25 additional
hours. The flower-stems were now cut off, for they do not secrete, and
the subterranean part of the plant was found to weigh 106.8 grams
(1611 grains), and the water secreted during the 48 hours weighed 11.9
grams (183 grains),—that is, one-ninth of the whole weight of the
plant, excluding the flower-stems. We should remember that plants in a
state of nature would probably secrete in 48 hours much more than the
above large amount, for their roots would continue all the time
absorbing sap from the plant on which they were parasitic.
With germinating monocotyledonous seeds, of which, however, we did not
observe a large number, the plumules, for instance, those of Asparagus
and Canna, are straight whilst breaking through the ground. With the
Gramineæ, the sheath-like cotyledons are likewise straight; they,
however, terminate in a sharp crest, which is white and somewhat
indurated; and this structure obviously facilitates their emergence
from the soil: the first true leaves escape from the sheath through a
slit beneath the chisel-like apex and at right angles to it. In the
case of the onion (Allium cepa) we again meet with an arch; the
leaf-like cotyledon being abruptly bowed, when it breaks through the
ground, with the apex still enclosed within the seed-coats. The crown
of the arch, as previously described, is developed into a white conical
protuberance, which we may safely believe to be a special adaptation
for this office.
The fact of so many organs of different kinds—hypocotyls and epicotyls,
the petioles of some cotyledons and of some first leaves, the
cotyledons of the onion, the rachis of some ferns, and some
flower-stems—being all arched whilst they break through the ground,
shows how just are Dr. Haberlandt’s[11] remarks on the importance of
the arch to seedling plants. He attributes its chief importance to the
upper, young, and more tender parts of the hypocotyl
or epicotyl, being thus saved from abrasion and pressure whilst
breaking through the ground. But we think that some importance may be
attributed to the increased force gained by the hypocotyl, epicotyl, or
other organ by being at first arched; for both legs of the arch
increase in length, and both have points of resistance as long as the
tip remains enclosed within the seed-coats; and thus the crown of the
arch is pushed up through the earth with twice as much force as that
which a straight hypocotyl, etc., could exert. As soon, however, as the
upper end has freed itself, all the work has to be done by the basal
leg. In the case of the epicotyl of the common bean, the basal leg (the
apex having freed itself from the seed-coats) grew upwards with a force
sufficient to lift a thin plate of zinc, loaded with 12 ounces. Two
more ounces were added, and the 14 ounces were lifted up to a very
little height, and then the epicotyl yielded and bent to one side.
[11] ‘Die Schutzeinrichtungen in der Entwickelung der Keimpflanze,’
1877. We have learned much from this interesting essay, though our
observations lead us to differ on some points from the author.
With respect to the primary cause of the arching process, we long
thought in the case of many seedlings that this might be attributed to
the manner in which the hypocotyl or epicotyl was packed and curved
within the seed-coats; and that the arched shape thus acquired was
merely retained until the parts in question reached the surface of the
ground. But it is doubtful whether this is the whole of the truth in
any case. For instance, with the common bean, the epicotyl or plumule
is bowed into an arch whilst breaking through the seed-coats, as shown
in Fig. 59 (p. 92). The plumule first protrudes as a solid knob (e in
A), which after twenty-four hours’ growth is seen (e in B) to be the
crown of an arch. Nevertheless, with several beans which germinated in
damp air, and had otherwise been treated in an unnatural manner, little
plumules were developed in the axils of the petioles of both
cotyledons, and these were as perfectly arched as the normal plumule;
yet they had not been subjected to any confinement or pressure, for the
seed-coats were completely ruptured, and they grew in the open air.
This proves that the plumule has an innate or spontaneous tendency to
arch itself.
In some other cases the hypocotyl or epicotyl protrudes from the seed
at first only slightly bowed; but the bowing afterwards increases
independently of any constraint. The arch is thus made narrow, with the
two legs, which are sometimes much elongated, parallel and close
together, and thus it becomes well fitted for breaking through the
ground.
With many kinds of plants, the radicle, whilst still enclosed within
the seed and likewise after its first protrusion, lies in a straight
line with the future hypocotyl and with the longitudinal axis of the
cotyledons. This is the case with Cucurbita ovifera: nevertheless, in
whatever position the seeds were buried, the hypocotyl always came up
arched in one particular direction. Seeds were planted in friable peat
at a depth of about an inch in a vertical position, with the end from
which the radicle protrudes downwards. Therefore all the parts occupied
the same relative positions which they would ultimately hold after the
seedlings had risen clear above the surface. Notwithstanding this fact,
the hypocotyl arched itself; and as the arch grew upwards through the
peat, the buried seeds were turned either upside down, or were laid
horizontally, being afterwards dragged above the ground. Ultimately the
hypocotyl straightened itself in the usual manner; and now after all
these movements the several parts occupied the same position relatively
to one another and to the centre of the earth, which they
had done when the seeds were first buried. But it may be argued in this
and other such cases that, as the hypocotyl grows up through the soil,
the seed will almost certainly be tilted to one side; and then from the
resistance which it must offer during its further elevation, the upper
part of the hypocotyl will be doubled down and thus become arched. This
view seems the more probable, because with Ranunculus ficaria only the
petioles of the leaves which forced a passage through the earth were
arched; and not those which arose from the summits of the bulbs above
the ground. Nevertheless, this explanation does not apply to the
Cucurbita, for when germinating seeds were suspended in damp air in
various positions by pins passing through the cotyledons, fixed to the
inside of the lids of jars, in which case the hypocotyls were not
subjected to any friction or constraint, yet the upper part became
spontaneously arched. This fact, moreover, proves that it is not the
weight of the cotyledons which causes the arching. Seeds of Helianthus
annuus and of two species of Ipomœa (those of ‘I. bona nox’ being for
the genus large and heavy) were pinned in the same manner, and the
hypocotyls became spontaneously arched; the radicles, which had been
vertically dependent, assumed in consequence a horizontal position. In
the case of Ipomœa leptophylla it is the petioles of the cotyledons
which become arched whilst rising through the ground; and this occurred
spontaneously when the seeds were fixed to the lids of jars.
It may, however, be suggested with some degree of probability that the
arching was aboriginally caused by mechanical compulsion, owing to the
confinement of the parts in question within the seed-coats, or to
friction whilst they were being dragged upwards. But
if this is so, we must admit from the cases just given, that a tendency
in the upper part of the several specified organs to bend downwards and
thus to become arched, has now become with many plants firmly
inherited. The arching, to whatever cause it may be due, is the result
of modified circumnutation, through increased growth along the convex
side of the part; such growth being only temporary, for the part always
straightens itself subsequently by increased growth along the concave
side, as will hereafter be described.
It is a curious fact that the hypocotyls of some plants, which are but
little developed and which never raise their cotyledons above the
ground, nevertheless inherit a slight tendency to arch themselves,
although this movement is not of the least use to them. We refer to a
movement observed by Sachs in the hypocotyls of the bean and some other
Leguminosae, and which is shown in the accompanying figure (Fig. 59),
copied from his Essay.[12] The hypocotyl and radicle at first grow
perpendicularly downwards, as at A, and then bend, often in the course
of 24 hours, into the position shown at B. As we shall hereafter often
have to recur to this movement, we will, for brevity sake, call it
“Sachs’ curvature.” At first sight it might be thought that the altered
position of the radicle in B was wholly due to the outgrowth of the
epicotyl (e), the petiole (p) serving as a hinge; and it is probable
that this is partly the cause; but the hypocotyl and upper part of the
radicle themselves become slightly curved.
[12] ‘Arbeiten des bot. Instit. Würzburg,’ vol. i. 1873, p. 403.
The above movement in the bean was repeatedly seen by us; but our
observations were made chiefly on Phaseolus multiflorus, the cotyledons
of which are
likewise hypogean. Some seedlings with well-developed radicles were
first immersed in a solution of permanganate of potassium; and, judging
from the changes of colour (though these were not very clearly
defined), the hypocotyl is about .3 inch in length. Straight, thin,
black lines of this length were now drawn from the bases of the short
petioles along the hypocotyls of 23 germinating seeds, which were
pinned to the lids of jars, generally with the hilum downwards, and
with their radicles pointing to the centre of the earth. After an
interval of from 24 to 48 hours the black lines on the hypocotyls of 16
out of the 23 seedlings became distinctly curved, but in very various
degrees (namely, with radii between 20 and
80 mm. on Sachs’ cyclometer) in the same relative direction as shown at
B in Fig. 59. As geotropism will obviously tend to check this
curvature, seven seeds were allowed to germinate with proper
precautions for their growth in a klinostat,[13] by which means
geotropism was eliminated. The position of the hypocotyls was observed
during four successive days, and they continued to bend towards the
hilum and lower surface of the seed. On the fourth day they were
deflected by an average angle of 63° from a line perpendicular to the
lower surface, and were therefore considerably more curved than the
hypocotyl and radicle in the bean at B (Fig. 59), though in the same
relative direction.
[13] An instrument devised by Sachs, consisting essentially of a
slowly revolving horizontal axis, on which the plant under observation
is supported: see ‘Würzburg Arbeiten,’ 1879, p. 209.
Fig. 59. Vicia faba: germinating seeds, suspended in damp air: A, with
radicle growing perpendicularly downwards; B, the same bean after 24
hours and after the radicle has curved itself; r. radicle; h, short
hypocotyl; e, epicotyl appearing as a knob in A and as an arch in B; p,
petiole of the cotyledon, the latter enclosed within the seed-coats.
It will, we presume, be admitted that all leguminous plants with
hypogean cotyledons are descended from forms which once raised their
cotyledons above the ground in the ordinary manner; and in doing so, it
is certain that their hypocotyls would have been abruptly arched, as in
the case of every other dicotyledonous plant. This is especially clear
in the case of Phaseolus, for out of five species, the seedlings of
which we observed, namely, P. multiflorus, caracalla, vulgaris,
Hernandesii and Roxburghii (inhabitants of the Old and New Worlds), the
three last-named species have well-developed hypocotyls which break
through the ground as arches. Now, if we imagine a seedling of the
common bean or of P. multiflorus, to behave as its progenitors once
did, the hypocotyl (h, Fig. 59), in whatever position the seed may have
been buried, would become so much arched that the upper part would be
doubled down parallel to the lower part; and
this is exactly the kind of curvature which actually occurs in these
two plants, though to a much less degree. Therefore we can hardly doubt
that their short hypocotyls have retained by inheritance a tendency to
curve themselves in the same manner as they did at a former period,
when this movement was highly important to them for breaking through
the ground, though now rendered useless by the cotyledons being
hypogean. Rudimentary structures are in most cases highly variable, and
we might expect that rudimentary or obsolete actions would be equally
so; and Sachs’ curvature varies extremely in amount, and sometimes
altogether fails. This is the sole instance known to us of the
inheritance, though in a feeble degree, of movements which have become
superfluous from changes which the species has undergone.
Rudimentary Cotyledons.—A few remarks on this subject may be here
interpolated. It is well known that some dicotyledonous plants produce
only a single cotyledon; for instance, certain species of Ranunculus,
Corydalis, Chaerophyllum; and we will here endeavour to show that the
loss of one or both cotyledons is apparently due to a store of
nutriment being laid up in some other part, as in the hypocotyl or one
of the two cotyledons, or one of the secondary radicles.
With the orange (Citrus aurantium) the cotyledons are hypogean, and one
is larger than the other, as may be seen in A (Fig. 60). In B the
inequality is rather greater, and the stem has grown between the points
of insertion of the two petioles, so that they do not stand opposite to
one another; in another case the separation amounted to one-fifth of an
inch. The smaller cotyledon of one seedling was extremely thin, and not
half the length of the larger one, so that it was clearly becoming
rudimentary[14] In all these seedlings the hypocotyl was enlarged or
swollen.
Fig. 60. Citrus aurantium: two young seedlings: c, larger cotyledon;
c’, smaller cotyledon; h, thickened hypocotyl; r, radicle. In A the
epicotyl is still arched, in B it has become erect.
Fig. 61. Abronia umbellata: seedling twice natural size: c cotyledon;
c’, rudimentary cotyledon; h, enlarged hypocotyl, with a heel or
projection (h’) at the lower end; r, radicle.
[14] In Pachira aquatica, as described by Mr. R. I. Lynch (‘Journal
Linn. Soc. Bot.’ vol. xvii. 1878, p. 147), one of the hypogean
cotyledons is of immense size; the other is small and soon falls off;
the pair do not always stand opposite. In another and very different
water-plant, ‘Trapa natans’, one of the cotyledons, filled with
farinaceous matter, is much larger than the other, which is scarcely
visible, as is stated by Aug. de Candolle, ‘Physiologie Veg.’ tom. ii.
p. 834, 1832.
With Abronia umbellata one of the cotyledons is quite rudimentary, as
may be seen (c’) in Fig. 61. In this specimen it consisted of a little
green flap, 1/84th inch in length, destitute of a petiole and covered
with glands like those on the fully developed cotyledon (c). At first
it stood opposite to the larger cotyledon; but as the petiole of the
latter increased in length and grew in the same line with the hypocotyl
(h), the rudiment appeared in older seedlings as if seated some way
down the hypocotyl. With Abronia arenaria there is a similar rudiment,
which in one
specimen was only 1/100th and in another 1/60th inch in length; it
ultimately appeared as if seated halfway down the hypocotyl. In both
these species the hypocotyl is so much enlarged, especially at a very
early age, that it might almost be called a corm. The lower end forms a
heel or projection, the use of which will hereafter be described.
In _Cyclamen Persicum_ the hypocotyl, even whilst still within the
seed, is enlarged into a regular corm,[15] and only a single cotyledon
is at first developed (see former Fig. 57). With _Ranunculus ficaria_
two cotyledons are never produced, and here one of the secondary
radicles is developed at an early age into a so-called bulb.[16] Again,
certain species of Chaerophyllum and Corydalis produce only a single
cotyledon;[17] in the former the hypocotyl, and in the latter the
radicle is enlarged, according to Irmisch, into a bulb.
[15] Dr. H. Gressner, ‘Bot. Zeitung,’ 1874, p. 824.
[16] Irmisch, ‘Beiträge zur Morphologie der Pflanzen,’ 1854, pp. 11,
12; ‘Bot. Zeitung,’ 1874, p. 805.
[17] Delpino, ‘Rivista Botanica,’ 1877, p. 21. It is evident from
Vaucher’s account (‘Hist. Phys. des Plantes d’Europe,’ tom. i. 1841,
p. 149) of the germination of the seeds of several species of
Corydalis, that the bulb or tubercule begins to be formed at an
extremely early age.
In the several foregoing cases one of the cotyledons is delayed in its
development, or reduced in size, or rendered rudimentary, or quite
aborted; but in other cases both cotyledons are represented by mere
rudiments. With Opuntia basilaris this is not the case, for both
cotyledons are thick and large, and the hypocotyl shows at first no
signs of enlargement; but afterwards, when the cotyledons have withered
and disarticulated themselves, it becomes thickened, and from its
tapering form, together with its smooth, tough, brown skin, appears,
when ultimately drawn down to some depth into the soil, like a root. On
the other
hand, with several other Cacteæ, the hypocotyl is from the first much
enlarged, and both cotyledons are almost or quite rudimentary. Thus
with Cereus Landbeckii two little triangular projections, representing
the cotyledons, are narrower than the hypocotyl, which is pear-shaped,
with the point downwards. In Rhipsalis cassytha the cotyledons are
represented by mere points on the enlarged hypocotyl. In Echinocactus
viridescens the hypocotyl is globular, with two little prominences on
its summit. In Pilocereus Houlletii the hypocotyl, much swollen in the
upper part, is merely notched on the summit; and each side of the notch
evidently represents a cotyledon. Stapelia sarpedon, a member of the
very distinct family of the Asclepiadeae, is fleshy like a cactus; and
here again the upper part of the flattened hypocotyl is much thickened
and bears two minute cotyledons, which, measured internally, were only
.15 inch in length, and in breadth not equal to one-fourth of the
diameter of the hypocotyl in its narrow axis; yet these minute
cotyledons are probably not quite useless, for when the hypocotyl
breaks through the ground in the form of an arch, they are closed or
pressed against one another, and thus protect the plumule. They
afterwards open.
From the several cases now given, which refer to widely distinct
plants, we may infer that there is some close connection between the
reduced size of one or both cotyledons and the formation, by the
enlargement of the hypocotyl or of the radicle, of a so-called bulb.
But it may be asked, did the cotyledons first tend to abort, or did a
bulb first begin to be formed? As all dicotyledons naturally produce
two well-developed cotyledons, whilst the thickness of the hypocotyl
and of the radicle differs much in different plants, it seems probable
that these latter organs first became from
some cause thickened—in several instances apparently in correlation
with the fleshy nature of the mature plant—so as to contain a store of
nutriment sufficient for the seedling, and then that one or both
cotyledons, from being superfluous, decreased in size. It is not
surprising that one cotyledon alone should sometimes have been thus
affected, for with certain plants, for instance the cabbage, the
cotyledons are at first of unequal size, owing apparently to the manner
in which they are packed within the seed. It does not, however, follow
from the above connection, that whenever a bulb is formed at an early
age, one or both cotyledons will necessarily become superfluous, and
consequently more or less rudimentary. Finally, these cases offer a
good illustration of the principle of compensation or balancement of
growth, or, as Goethe expresses it, “in order to spend on one side,
Nature is forced to economise on the other side.”
Circumnutation and other movements of Hypocotyls and Epicotyls, whilst
still arched and buried beneath the ground, and whilst breaking through
it.—According to the position in which a seed may chance to have been
buried, the arched hypocotyl or epicotyl will begin to protrude in a
horizontal, a more or less inclined, or in a vertical plane. Except
when already standing vertically upwards, both legs of the arch are
acted on from the earliest period by apogeotropism. Consequently they
both bend upwards until the arch becomes vertical. During the whole of
this process, even before the arch has broken through the ground, it is
continually trying to circumnutate to a slight extent; as it likewise
does if it happens at first to stand vertically up,—all which cases
have been observed and described, more or less fully, in the last
chapter. After the arch has grown to some
height upwards the basal part ceases to circumnutate, whilst the upper
part continues to do so.
That an arched hypocotyl or epicotyl, with the two legs fixed in the
ground, should be able to circumnutate, seemed to us, until we had read
Prof. Wiesner’s observations, an inexplicable fact. He has shown[18] in
the case of certain seedlings, whose tips are bent downwards (or which
nutate), that whilst the posterior side of the upper or dependent
portion grows quickest, the anterior and opposite side of the basal
portion of the same internode grows quickest; these two portions being
separated by an indifferent zone, where the growth is equal on all
sides. There may be even more than one indifferent zone in the same
internode; and the opposite sides of the parts above and below each
such zone grow quickest. This peculiar manner of growth is called by
Wiesner “undulatory nutation.” Circumnutation depends on one side of an
organ growing quickest (probably preceded by increased turgescence),
and then another side, generally almost the opposite one, growing
quickest. Now if we look at an arch like this [upside down U] and
suppose the whole of one side—we will say the whole convex side of both
legs—to increase in length, this would not cause the arch to bend to
either side. But if the outer side or surface of the left leg were to
increase in length the arch would be pushed over to the right, and this
would be aided by the inner side of the right leg increasing in length.
If afterwards the process were reversed, the arch would be pushed over
to the opposite or left side, and so on alternately,—that is, it would
circumnutate. As an arched
hypocotyl, with the two legs fixed in the ground, certainly
circumnutates, and as it consists of a single internode, we may
conclude that it grows in the manner described by Wiesner. It may be
added, that the crown of the arch does not grow, or grows very slowly,
for it does not increase much in breadth, whilst the arch itself
increases greatly in height.
[18] ‘Die undulirende Nutation der Internodien,’ Akad. der Wissench.
(Vienna), Jan. 17th, 1878. Also published separately, see p. 32.
The circumnutating movements of arched hypocotyls and epicotyls can
hardly fail to aid them in breaking through the ground, if this be damp
and soft; though no doubt their emergence depends mainly on the force
exerted by their longitudinal growth. Although the arch circumnutates
only to a slight extent and probably with little force, yet it is able
to move the soil near the surface, though it may not be able to do so
at a moderate depth. A pot with seeds of Solanum palinacanthum, the
tall arched hypocotyls of which had emerged and were growing rather
slowly, was covered with fine argillaceous sand kept damp, and this at
first closely surrounded the bases of the arches; but soon a narrow
open crack was formed round each of them, which could be accounted for
only by their having pushed away the sand on all sides; for no such
cracks surrounded some little sticks and pins which had been driven
into the sand. It has already been stated that the cotyledons of
Phalaris and Avena, the plumules of Asparagus and the hypocotyls of
Brassica, were likewise able to displace the same kind of sand, either
whilst simply circumnutating or whilst bending towards a lateral light.
As long as an arched hypocotyl or epicotyl remains buried beneath the
ground, the two legs cannot separate from one another, except to a
slight extent from the yielding of the soil; but as soon as the arch
rises above the ground, or at an earlier period if
the pressure of the surrounding earth be artificially removed, the arch
immediately begins to straighten itself. This no doubt is due to growth
along the whole inner surface of both legs of the arch; such growth
being checked or prevented, as long as the two legs of the arch are
firmly pressed together. When the earth is removed all round an arch
and the two legs are tied together at their bases, the growth on the
under side of the crown causes it after a time to become much flatter
and broader than naturally occurs. The straightening process consists
of a modified form of circumnutation, for the lines described during
this process (as with the hypocotyl of Brassica, and the epicotyls of
Vicia and Corylus) were often plainly zigzag and sometimes looped.
After hypocotyls or epicotyls have emerged from the ground, they
quickly become perfectly straight. No trace is left of their former
abrupt curvature, excepting in the case of Allium cepa, in which the
cotyledon rarely becomes quite straight, owing to the protuberance
developed on the crown of the arch.
The increased growth along the inner surface of the arch which renders
it straight, apparently begins in the basal leg or that which is united
to the radicle; for this leg, as we often observed, is first bowed
backwards from the other leg. This movement facilitates the withdrawal
of the tip of the epicotyl or of the cotyledons, as the case may be,
from within the seed-coats and from the ground. But the cotyledons
often emerge from the ground still tightly enclosed within the
seed-coats, which apparently serve to protect them. The seed-coats are
afterwards ruptured and cast off by the swelling of the closely
conjoined cotyledons, and not by any movement or their separation from
one another.
Nevertheless, in some few cases, especially with the
Cucurbitaceæ, the seed-coats are ruptured by a curious contrivance,
described by M. Flahault.[19] A heel or peg is developed on one side of
the summit of the radicle or base of the hypocotyl; and this holds down
the lower half of the seed-coats (the radicle being fixed into the
ground) whilst the continued growth of the arched hypocotyl forced
upwards the upper half, and tears asunder the seed-coats at one end,
and the cotyledons are then easily withdrawn.
[19] ‘Bull. Soc. Bot. de France,’ tom. xxiv. 1877, p. 201.
The accompanying figure (Fig. 62) will render this description
intelligible. Forty-one seeds of Cucurbita ovifera were laid on friable
peat and were covered by a layer about an inch in thickness, not much
pressed down, so that the cotyledons in being dragged up were subjected
to very little friction, yet forty of them came up naked, the
seed-coats being left buried in the peat. This was certainly due to the
action of the peg, for when it was prevented from acting, the
cotyledons, as we shall presently see, were lifted up still enclosed in
their seed-coats. They were, however, cast off in the course of two or
three days by the swelling of the cotyledons. Until this occurs light
is excluded, and the cotyledons cannot decompose carbonic acid; but no
one probably would have thought that the advantage thus gained by a
little earlier
casting off of the seed-coats would be sufficient to account for the
development of the peg. Yet according to M. Flahault, seedlings which
have been prevented from casting their seed-coats whilst beneath the
ground, are inferior to those which have emerged with their cotyledons
naked and ready to act.
Fig. 62. Cucurbita ovifera: germinating seed, showing the heel or peg
projecting on one side from summit of radicle and holding down lower
tip of seed-coats, which have been partially ruptured by the growth of
the arched hypocotyl.
The peg is developed with extraordinary rapidity; for it could only
just be distinguished in two seedlings, having radicles .35 inch in
length, but after an interval of only 24 hours was well developed in
both. It is formed, according to Flahault, by the enlargement of the
layers of the cortical parenchyma at the base of the hypocotyl. If,
however, we judge by the effects of a solution of permanganate of
potassium, it is developed on the exact line of junction between the
hypocotyl and radicle; for the flat lower surface, as well as the
edges, were coloured brown like the radicle; whilst the upper slightly
inclined surface was left uncoloured like the hypocotyl, excepting
indeed in one out of 33 immersed seedlings in which a large part of the
upper surface was coloured brown. Secondary roots sometimes spring from
the lower surface of the peg, which thus seems in all respects to
partake of the nature of the radicle. The peg is always developed on
the side which becomes concave by the arching of the hypocotyl; and it
would be of no service if it were formed on any other side. It is also
always developed with the flat lower side, which, as just stated, forms
a part of the radicle, at right angles to it, and in a horizontal
plane. This fact was clearly shown by burying some of the thin flat
seeds in the same position as in Fig. 62, excepting that they were not
laid on their flat broad sides, but with one edge downwards. Nine seeds
were thus planted, and the peg was developed in the
same position, relatively to the radicle, as in the figure;
consequently it did not rest on the flat tip of the lower half of the
seed-coats, but was inserted like a wedge between the two tips. As the
arched hypocotyl grew upwards it tended to draw up the whole seed, and
the peg necessarily rubbed against both tips, but did not hold either
down. The result was, that the cotyledons of five out of the nine seeds
thus placed were raised above the ground still enclosed within their
seed-coats. Four seeds were buried with the end from which the radicle
protrudes pointing vertically downwards, and owing to the peg being
always developed in the same position, its apex alone came into contact
with, and rubbed against the tip on one side; the result was, that the
cotyledons of all four emerged still within their seed-coats. These
cases show us how the peg acts in co-ordination with the position which
the flat, thin, broad seeds would almost always occupy when naturally
sown. When the tip of the lower half of the seed-coats was cut off,
Flahault found (as we did likewise) that the peg could not act, since
it had nothing to press on, and the cotyledons were raised above the
ground with their seed-coats not cast off. Lastly, nature shows us the
use of the peg; for in the one Cucurbitaceous genus known to us, in
which the cotyledons are hypogean and do not cast their seed-coats,
namely, Megarrhiza, there is no vestige of a peg. This structure seems
to be present in most of the other genera in the family, judging from
Flahault’s statements’ we found it well-developed and properly acting
in Trichosanthes anguina, in which we hardly expected to find it, as
the cotyledons are somewhat thick and fleshy. Few cases can be advanced
of a structure better adapted for a special purpose than the present
one.
With Mimosa pudica the radicle protrudes from a small hole in the sharp
edge of the seed; and on its summit, where united with the hypocotyl, a
transverse ridge is developed at an early age, which clearly aids in
splitting the tough seed-coats; but it does not aid in casting them
off, as this is subsequently effected by the swelling of the cotyledons
after they have been raised above the ground. The ridge or heel
therefore acts rather differently from that of Cucurbita. Its lower
surface and the edges were coloured brown by the permanganate of
potassium, but not the upper surface. It is a singular fact that after
the ridge has done its work and has escaped from the seed-coats, it is
developed into a frill all round the summit of the radicle.[20]
[20] Our attention was called to this case by a brief statement by
Nobbe in his ‘Handbuch der Samenkunde,’ 1876, p. 215, where a figure
is also given of a seedling of Martynia with a heel or ridge at the
junction of the radicle and hypocotyl. This seed possesses a very hard
and tough coat, and would be likely to require aid in bursting and
freeing the cotyledons.
At the base of the enlarged hypocotyl of Abronia umbellata, where it
blends into the radicle, there is a projection or heel which varies in
shape, but its outline is too angular in our former figure (Fig. 61).
The radicle first protrudes from a small hole at one end of the tough,
leathery, winged fruit. At this period the upper part of the radicle is
packed within the fruit parallel to the hypocotyl, and the single
cotyledon is doubled back parallel to the latter. The swelling of these
three parts, and especially the rapid development of the thick heel
between the hypocotyl and radicle at the point where they are doubled,
ruptures the tough fruit at the upper end and allows the arched
hypocotyl to emerge; and this seems to be the function of the heel. A
seed was cut out of the fruit and
allowed to germinate in damp air, and now a thin flat disc was
developed all round the base of the hypocotyl and grew to an
extraordinary breadth, like the frill described under Mimosa, but
somewhat broader. Flahault says that with Mirabilis, a member of the
same family with Abronia, a heel or collar is developed all round the
base of the hypocotyl, but more on one side than on the other; and that
it frees the cotyledons from their seed-coats. We observed only old
seeds, and these were ruptured by the absorption of moisture,
independently of any aid from the heel and before the protrusion of the
radicle; but it does not follow from our experience that fresh and
tough fruits would behave in a like manner.
In concluding this section of the present chapter it may be convenient
to summarise, under the form of an illustration, the usual movements of
the hypocotyls and epicotyls of seedlings, whilst breaking through the
ground and immediately afterwards. We may suppose a man to be thrown
down on his hands and knees, and at the same time to one side, by a
load of hay falling on him. He would first endeavour to get his arched
back upright, wriggling at the same time in all directions to free
himself a little from the surrounding pressure; and this may represent
the combined effects of apogeotropism and circumnutation, when a seed
is so buried that the arched hypocotyl or epicotyl protrudes at first
in a horizontal or inclined plane. The man, still wriggling, would then
raise his arched back as high as he could; and this may represent the
growth and continued circumnutation of an arched hypocotyl or epicotyl,
before it has reached the surface of the ground. As soon as the man
felt himself at all free, he would raise the upper part of his body,
whilst still on
his knees and still wriggling; and this may represent the bowing
backwards of the basal leg of the arch, which in most cases aids in the
withdrawal of the cotyledons from the buried and ruptured seed-coats,
and the subsequent straightening of the whole hypocotyl or
epicotyl—circumnutation still continuing.
Circumnutation of Hypocotyls and Epicotyls, when erect.—The hypocotyls,
epicotyls, and first shoots of the many seedlings observed by us, after
they had become straight and erect, circumnutated continuously. The
diversified figures described by them, often during two successive
days, have been shown in the woodcuts in the last chapter. It should be
recollected that the dots were joined by straight lines, so that the
figures are angular; but if the observations had been made every few
minutes the lines would have been more or less curvilinear, and
irregular ellipses or ovals, or perhaps occasionally circles, would
have been formed. The direction of the longer axes of the ellipses made
during the same day or on successive days generally changed completely,
so as to stand at right angles to one another. The number of irregular
ellipses or circles made within a given time differs much with
different species. Thus with Brassica oleracea, Cerinthe major, and
Cucurbita ovifera about four such figures were completed in 12 h.;
whereas with Solanum palinacanthum and Opuntia basilaris, scarcely more
than one. The figures likewise differ greatly in size; thus they were
very small and in some degree doubtful in Stapelia, and large in
Brassica, etc. The ellipses described by Lathyrus nissolia and Brassica
were narrow, whilst those made by the Oak were broad. The figures are
often complicated by small loops and zigzag lines.
As most seedling plants before the development of true leaves are of
low, sometimes very low stature,
the extreme amount of movement from side to side of their
circumnutating stems was small; that of the hypocotyl of Githago
segetum was about .2 of an inch, and that of Cucurbita ovifera about
.28. A very young shoot of Lathyrus nissolia moved about .14, that of
an American oak .2, that of the common nut only .04, and a rather tall
shoot of the Asparagus .11 of an inch. The extreme amount of movement
of the sheath-like cotyledon of Phalaris Canariensis was .3 of an inch;
but it did not move very quickly, the tip crossing on one occasion five
divisions of the micrometer, that is, 1/100th of an inch, in 22 m. 5 s.
A seedling Nolana prostrata travelled the same distance in 10 m. 38 s.
Seedling cabbages circumnutate much more quickly, for the tip of a
cotyledon crossed 1/100th of an inch on the micrometer in 3 m. 20 s.;
and this rapid movement, accompanied by incessant oscillations, was a
wonderful spectacle when beheld under the microscope.
The absence of light, for at least a day, does not interfere in the
least with the circumnutation of the hypocotyls, epicotyls, or young
shoots of the various dicotyledonous seedlings observed by us; nor with
that of the young shoots of some monocotyledons. The circumnutation was
indeed much plainer in darkness than in light, for if the light was at
all lateral the stem bent towards it in a more or less zigzag course.
Finally, the hypocotyls of many seedlings are drawn during the winter
into the ground, or even beneath it so that they disappear. This
remarkable process, which apparently serves for their protection, has
been fully described by De Vries.[21] He shows that
it is effected by the contraction of the parenchyma-cells of the root.
But the hypocotyl itself in some cases contracts greatly, and although
at first smooth becomes covered with zigzag ridges, as we observed with
Githago segetum. How much of the drawing down and burying of the
hypocotyl of Opuntia basilaris was due to the contraction of this part
and how much to that of the radicle, we did not observe.
[21] ‘Bot. Zeitung,’ 1879, p. 649. See also Winkler in ‘Verhandl. des
Bot. Vereins der P. Brandenburg,’ Jahrg. xvi. p. 16, as quoted by
Haberlandt, ‘Schutzeinrichungen der Keimpflanze,’ 1877, p. 52.
Circumnutation of Cotyledons.—With all the dicotyledonous seedlings
described in the last chapter, the cotyledons were in constant
movement, chiefly in a vertical plane, and commonly once up and once
down in the course of the 24 hours. But there were many exceptions to
such simplicity of movement; thus the cotyledons of Ipomœa caerulea
moved 13 times either upwards or downwards in the course of 16 h.. 18
m. Those of Oxalis rosea moved in the same manner 7 times in the course
of 24 h.; and those of Cassia tora described 5 irregular ellipses in 9
h. The cotyledons of some individuals of Mimosa pudica and of Lotus
Jacobæus moved only once up and down in 24 h., whilst those of others
performed within the same period an additional small oscillation. Thus
with different species, and with different individuals of the same
species, there were many gradations from a single diurnal movement to
oscillations as complex as those of the Ipomœa and Cassia. The opposite
cotyledons on the same seedling move to a certain extent independently
of one another. This was conspicuous with those of Oxalis sensitiva, in
which one cotyledon might be seen during the daytime rising up until it
stood vertically, whilst the opposite one was sinking down.
Although the movements of cotyledons were generally in nearly the same
vertical plane, yet their upward and downward courses never exactly
coincided; so that ellipses, more or less narrow, were described, and
the cotyledons may safely be said to have circumnutated. Nor could this
fact be accounted for by the mere increase in length of the cotyledons
through growth, for this by itself would not induce any lateral
movement. That there was lateral movement in some instances, as with
the cotyledons of the cabbage, was evident; for these, besides moving
up and down, changed their course from right to left 12 times in 14 h.
15 m. With Solanum lycopersicum the cotyledons, after falling in the
forenoon, zigzagged from side to side between 12 and 4 P.M., and then
commenced rising. The cotyledons of Lupinus luteus are so thick (about
.08 of an inch) and fleshy,[22] that they seemed little likely to move,
and were therefore observed with especial interest; they certainly
moved largely up and down, and as the line traced was zigzag there was
some lateral movement. The nine cotyledons of a seedling Pinus pinaster
plainly circumnutated; and the figures described approached more nearly
to irregular circles than to irregular ovals or ellipses. The
sheath-like cotyledons of the Gramineæ circumnutate, that is, move to
all sides, as plainly as do the hypocotyls or epicotyls of any
dicotyledonous plants. Lastly, the very young fronds of a Fern and of a
Selaginella circumnutated.
[22] The cotyledons, though bright green, resemble to a certain extent
hypogean ones; see the interesting discussion by Haberlandt (‘Die
Schutzeinrichtungen,’ etc., 1877, p. 95), on the gradations in the
Leguminosae between subaërial and subterranean cotyledons.
In a large majority of the cases which were carefully observed, the
cotyledons sink a little downwards in the forenoon, and rise a little
in the afternoon or evening. They thus stand rather more highly
inclined during the night than during the mid-day, at which
time they are expanded almost horizontally. The circumnutating movement
is thus at least partially periodic, no doubt in connection, as we
shall hereafter see, with the daily alternations of light and darkness.
The cotyledons of several plants move up so much at night as to stand
nearly or quite vertically; and in this latter case they come into
close contact with one another. On the other hand, the cotyledons of a
few plants sink almost or quite vertically down at night; and in this
latter case they clasp the upper part of the hypocotyl. In the same
genus Oxalis the cotyledons of certain species stand vertically up, and
those of other species vertically down, at night. In all such cases the
cotyledons may be said to sleep, for they act in the same manner as do
the leaves of many sleeping plants. This is a movement for a special
purpose, and will therefore be considered in a future chapter devoted
to this subject.
In order to gain some rude notion of the proportional number of cases
in which the cotyledons of dicotyledonous plants (hypogean ones being
of course excluded) changed their position in a conspicuous manner at
night, one or more species in several genera were cursorily observed,
besides those described in the last chapter. Altogether 153 genera,
included in as many families as could be procured, were thus observed
by us. The cotyledons were looked at in the middle of the day and again
at night; and those were noted as sleeping which stood either
vertically or at an angle of at least 60° above or beneath the horizon.
Of such genera there were 26; and in 21 of them the cotyledons of some
of the species rose, and in only 6 sank at night; and some of these
latter cases are rather doubtful from causes to be explained in the
chapter on the sleep of cotyledons. When
cotyledons which at noon were nearly horizontal, stood at night at more
than 20° and less than 60° above the horizon, they were recorded as
“plainly raised;” and of such genera there were 38. We did not meet
with any distinct instances of cotyledons periodically sinking only a
few degrees at night, although no doubt such occur. We have now
accounted for 64 genera out of the 153, and there remain 89 in which
the cotyledons did not change their position at night by as much as
20°—that is, in a conspicuous manner which could easily be detected by
the unaided eye and by memory; but it must not be inferred from this
statement that these cotyledons did not move at all, for in several
cases a rise of a few degrees was recorded, when they were carefully
observed. The number 89 might have been a little increased, for the
cotyledons remained almost horizontal at night in some species in a few
genera, for instance, Trifolium and Geranium, which are included
amongst the sleepers, such genera might therefore have been added to
the 89. Again, one species of Oxalis generally raised its cotyledons at
night more than 20° and less than 60° above the horizon; so that this
genus might have been included under two heads. But as several species
in the same genus were not often observed, such double entries have
been avoided.
In a future chapter it will be shown that the leaves of many plants
which do not sleep, rise a few degrees in the evening and during the
early part of the night; and it will be convenient to defer until then
the consideration of the periodicity of the movements of cotyledons.
On the Pulvini or Joints of Cotyledons.—With several of the seedlings
described in this and the last chapter, the summit of the petiole is
developed into a pulvinus,
cushion, or joint (as this organ has been variously called), like that
with which many leaves are provided. It consists of a mass of small
cells usually of a pale colour from the absence of chlorophyll, and
with its outline more or less convex, as shown in the annexed figure.
In the case of Oxalis sensitiva two-thirds of the petiole, and in that
of Mimosa pudica, apparently the whole of the short sub-petioles of the
leaflets have been converted into pulvini. With pulvinated leaves (i.e.
those provided with a pulvinus) their periodical movements depend,
according to Pfeffer,[23] on the cells of the pulvinus alternately
expanding more quickly on one side than on the other; whereas the
similar movements of leaves not provided with pulvini, depend on their
growth being alternately more rapid on one side than on the other.[24]
As long as a leaf provided with a pulvinus is young and continues to
grow, its movement depends on both these causes combined;[25] and if
the view now held by many botanists be sound, namely, that growth is
always preceded by the expansion of the growing cells, then the
difference between the movements induced by the aid of pulvini and
without such aid, is reduced to the expansion of the cells not being
followed by growth in the first case, and being so followed in the
second case.
[23] ‘Die Periodische Bewegungen der Blattorgane,’ 1875.
[24] Batalin, ‘Flora,’ Oct. 1st, 1873
[25] Pfeffer, ibid. p. 5.
Fig. 63. Oxalis rosea: longitudinal section of a pulvinus on the summit
of the petiole of a cotyledon, drawn with the camera lucida, magnified
75 times: p, p, petiole; f, fibro-vascular bundle: b, b, commencement
of blade of cotyledon.
Dots were made with Indian ink along the midrib of both pulvinated
cotyledons of a rather old seedling of Oxalis Valdiviana; their
distances were repeatedly measured with an eye-piece micrometer during
8 3/4 days, and they did not exhibit the least trace of increase. It is
therefore almost certain that the pulvinus itself was not then growing.
Nevertheless, during this whole time and for ten days afterwards, these
cotyledons rose vertically every night. In the case of some seedlings
raised from seeds purchased under the name of Oxalis floribunda, the
cotyledons continued for a long time to move vertically down at night,
and the movement apparently depended exclusively on the pulvini, for
their petioles were of nearly the same length in young, and in old
seedlings which had produced true leaves. With some species of Cassia,
on the other hand, it was obvious without any measurement that the
pulvinated cotyledons continued to increase greatly in length during
some weeks; so that here the expansion of the cells of the pulvini and
the growth of the petiole were probably combined in causing their
prolonged periodic movements. It was equally evident that the
cotyledons of many plants, not provided with pulvini, increased rapidly
in length; and their periodic movements no doubt were exclusively due
to growth.
In accordance with the view that the periodic movements of all
cotyledons depend primarily on the expansion of the cells, whether or
not followed by growth, we can understand the fact that there is but
little difference in the kind or form of movement in the two sets of
cases. This may be seen by
comparing the diagrams given in the last chapter. Thus the movements of
the cotyledons of Brassica oleracea and of Ipomœa caerulea, which are
not provided with pulvini, are as complex as those of Oxalis and Cassia
which are thus provided. The pulvinated cotyledons of some individuals
of Mimosa pudica and Lotus Jacobæus made only a single oscillation,
whilst those of other individuals moved twice up and down in the course
of 24 hours; so it was occasionally with the cotyledons of Cucurbita
ovifera, which are destitute of a pulvinus. The movements of pulvinated
cotyledons are generally larger in extent than those without a
pulvinus; nevertheless some of the latter moved through an angle of
90°. There is, however, one important difference in the two sets of
cases; the nocturnal movements of cotyledons without pulvini, for
instance, those in the Cruciferae, Cucurbitaceæ, Githago, and Beta,
never last even for a week, to any conspicuous degree. Pulvinated
cotyledons, on the other hand, continue to rise at night for a much
longer period, even for more than a month, as we shall now show. But
the period no doubt depends largely on the temperature to which the
seedlings are exposed and their consequent rate of development.
Oxalis Valdiviana.—Some cotyledons which had lately opened and were
horizontal on March 6th at noon, stood at night vertically up; on the
13th the first true leaf was formed, and was embraced at night by the
cotyledons; on April 9th, after an interval of 35 days, six leaves were
developed, and yet the cotyledons rose almost vertically at night. The
cotyledons of another seedling, which when first observed had already
produced a leaf, stood vertically at night and continued to do so for
11 additional days. After 16 days from the first observation two leaves
were developed, and the cotyledons were still greatly raised at night.
After 21 days the cotyledons during the day were deflected beneath the
horizon, but at night were raised 45°
above it. After 24 days from the first observation (begun after a true
leaf had been developed) the cotyledons ceased to rise at night.
Oxalis (Biophytum) sensitiva.—The cotyledons of several seedlings, 45
days after their first expansion, stood nearly vertical at night, and
closely embraced either one or two true leaves which by this time had
been formed. These seedlings had been kept in a very warm house, and
their development had been rapid.
Oxalis corniculata.—The cotyledons do not stand vertical at night, but
generally rise to an angle of about 45° above the horizon. They
continued thus to act for 23 days after their first expansion, by which
time two leaves had been formed; even after 29 days they still rose
moderately above their horizontal or downwardly deflected diurnal
position.
Mimosa pudica.—The cotyledons were expanded for the first time on Nov.
2nd, and stood vertical at night. On the 15th the first leaf was
formed, and at night the cotyledons were vertical. On the 28th they
behaved in the same manner. On Dec. 15th, that is after 44 days, the
cotyledons were still considerably raised at night; but those of
another seedling, only one day older, were raised very little.
Mimosa albida.—A seedling was observed during only 12 days, by which
time a leaf had been formed, and the cotyledons were then quite
vertical at night.
Trifolium subterraneum.—A seedling, 8 days old, had its cotyledons
horizontal at 10.30 A.M. and vertical at 9.15 P.M. After an interval of
two months, by which time the first and second true leaves had been
developed, the cotyledons still performed the same movement. They had
now increased greatly in size, and had become oval; and their petioles
were actually .8 of an inch in length!
Trifolium strictum.—After 17 days the cotyledons still rose at night,
but were not afterwards observed.
Lotus Jacoboeus.—The cotyledons of some seedlings having well-developed
leaves rose to an angle of about 45° at night; and even after 3 or 4
whorls of leaves had been formed, the cotyledons rose at night
considerably above their diurnal horizontal position.
Cassia mimosoides.—The cotyledons of this Indian species, 14 days after
their first expansion, and when a leaf had been formed, stood during
the day horizontal, and at night vertical.
Cassia sp? (a large S. Brazilian tree raised from seeds sent us
by F. Müller).—The cotyledons, after 16 days from their first
expansion, had increased greatly in size with two leaves just formed.
They stood horizontally during the day and vertically at night, but
were not afterwards observed.
Cassia neglecta (likewise a S. Brazilian species).—A seedling, 34 days
after the first expansion of its cotyledons, was between 3 and 4 inches
in height, with 3 well-developed leaves; and the cotyledons, which
during the day were nearly horizontal, at night stood vertical, closely
embracing the young stem. The cotyledons of another seedling of the
same age, 5 inches in height, with 4 well-developed leaves, behaved at
night in exactly the same manner.
It is known[26] that there is no difference in structure between the
upper and lower halves of the pulvini of leaves, sufficient to account
for their upward or downward movements. In this respect cotyledons
offer an unusually good opportunity for comparing the structure of the
two halves; for the cotyledons of Oxalis Valdiviana rise vertically at
night, whilst those of O. rosea sink vertically; yet when sections of
their pulvini were made, no clear difference could be detected between
the corresponding halves of this organ in the two species which move so
differently. With O. rosea, however, there were rather more cells in
the lower than in the upper half, but this was likewise the case in one
specimen of O. Valdiviana. the cotyledons of both species (3½ mm. in
length) were examined in the morning whilst extended horizontally, and
the upper surface of the pulvinus of O. rosea was then wrinkled
transversely, showing that it was in a state of compression, and this
might have been expected, as the cotyledons sink at night; with O.
Valdiviana it was the lower surface which was wrinkled, and its
cotyledons rise at night.
[26] Pfeffer, ‘Die Period. Bewegungen,’ 1875, p. 157.
Trifolium is a natural genus, and the leaves of all
the species seen by us are pulvinated; so it is with the cotyledons of
T. subterraneum and strictum, which stand vertically at night; whereas
those of T. resupinatum exhibit not a trace of a pulvinus, nor of any
nocturnal movement. This was ascertained by measuring the distance
between the tips of the cotyledons of four seedlings at mid-day and at
night. In this species, however, as in the others, the first-formed
leaf, which is simple or not trifoliate, rises up and sleeps like the
terminal leaflet on a mature plant.
In another natural genus, Oxalis, the cotyledons of O. Valdiviana,
rosea, floribunda, articulata, and sensitiva are pulvinated, and all
move at night into an upward or downward vertical position. In these
several species the pulvinus is seated close to the blade of the
cotyledon, as is the usual rule with most plants. Oxalis corniculata
(var. Atro-purpurea) differs in several respects; the cotyledons rise
at night to a very variable amount, rarely more than 45°; and in one
lot of seedlings (purchased under the name of O. tropaeoloides, but
certainly belonging to the above variety) they rose only from 5° to 15°
above the horizon. The pulvinus is developed imperfectly and to an
extremely variable degree, so that apparently it is tending towards
abortion. No such case has hitherto, we believe, been described. It is
coloured green from its cells containing chlorophyll; and it is seated
nearly in the middle of the petiole, instead of at the upper end as in
all the other species. The nocturnal movement is effected partly by its
aid, and partly by the growth of the upper part of the petiole as in
the case of plants destitute of a pulvinus. From these several reasons
and from our having partially traced the development of the pulvinus
from an early age, the case seems worth describing in some detail.
When the cotyledons of O. corniculata were dissected out of a seed from
which they would soon have naturally emerged, no trace of a pulvinus
could be detected; and all the cells forming the short petiole, 7 in
number in a longitudinal row, were of nearly equal size. In seedlings
one or two days old, the pulvinus was so indistinct that we thought at
first that it did not exist; but in the middle of the petiole an
ill-defined transverse zone of cells could be seen, which were much
shorter than those both above and below, although of the same breadth
with them. They presented the appearance of having been just formed by
the transverse division of longer cells; and there can be little doubt
that this had occurred, for the cells in the petiole which had been
dissected out of the seed averaged in length 7 divisions of the
micrometer (each division equalling .003 mm.), and were a little longer
than those forming a well-developed pulvinus, which varied between 4
and 6 of these same divisions. After a few additional days the
ill-defined zone of cells becomes distinct, and although it does not
extend across the whole width of the petiole, and although the cells
are of a green colour from containing chlorophyll, yet they certainly
constitute a pulvinus, which as we shall presently see, acts as one.
These small cells were arranged in longitudinal rows, and varied from 4
to 7 in number; and the cells themselves varied in length in different
parts of the
same pulvinus and in different individuals. In the accompanying
figures, A and B (Fig. 64), we have views of the epidermis[27] in the
middle part of the petioles of two seedlings, in which the pulvinus was
for this species well developed. They offer a striking contrast with
the pulvinus of O. rosea (see former Fig. 63), or of O. Valdiviana.
With the seedlings, falsely called O. tropaeoloides, the cotyledons of
which rise very little at night, the small cells were still fewer in
number and in parts formed a single transverse row, and in other parts
short longitudinal rows of only two or three. Nevertheless they
sufficed to attract the eye, when the whole petiole was viewed as a
transparent object beneath the microscope. In these seedlings there
could hardly be a doubt that the pulvinus was becoming rudimentary and
tending to disappear; and this accounts for its great variability in
structure and function.
[27] Longitudinal sections show that the forms of the epidermic cells
may be taken as a fair representation of those constituting the
pulvinus.
Fig. 64. Oxalis corniculata: A and B the almost rudimentary pulvini of
the cotyledons of two rather old seedlings, viewed as transparent
objects. Magnified 50 times.
In the following Table some measurements of the cells in fairly
well-developed pulvini of O. corniculata are given:—
Seedling 1 day old, with cotyledon 2.3 mm. in length. Divisions of
Micrometer.[28] Average length of cells of
pulvinus..................................................6 to 7 Length
of longest cell below the pulvinus.....................................
13 Length of longest cell above the
pulvinus...................................... 20
Seedling 5 days old, cotyledon 3.1 mm. in length, with the pulvinus
quite distinct. Average length of cells of
pulvinus.................................................. 6 Length of
longest cell below the pulvinus..................................... 22
Length of longest cell above the
pulvinus...................................... 40
Seedling 8 days old, cotyledon 5 mm. in length, with a true leaf formed
but not yet expanded. Average length of cells of
pulvinus.................................................. 9 Length of
longest cell below the pulvinus..................................... 44
Length of longest cell above the
pulvinus...................................... 70
Seedling 13 days old, cotyledon 4.5 mm. in length, with a small true
leaf fully developed. Average length of cells of
pulvinus.................................................. 7 Length of
longest cell below the pulvinus..................................... 30
Length of longest cell above the
pulvinus...................................... 60
[28] Each division equalled .003 mm.
We here see that the cells of the pulvinus increase but little in
length with advancing age, in comparison with those of the petiole both
above and below it; but they continue to grow in width, and keep equal
in this respect with the other cells of the petiole. The rate of
growth, however, varies in all parts of the cotyledons, as may be
observed in the measurements of the 8-days’ old seedling.
The cotyledons of seedlings only a day old rise at night considerably,
sometimes as much as afterwards; but there was much variation in this
respect. As the pulvinus is so indistinct at first, the movement
probably does not then depend on the expansion of its cells, but on
periodically unequal growth in the petiole. By the comparison of
seedlings of different known ages, it was evident that the chief seat
of growth of the petiole was in the upper part between the pulvinus and
the blade; and this agrees with the fact (shown in the measurements
above given) that the cells grow to a greater length in the upper than
in the lower part. With a seedling 11 days old, the nocturnal rise was
found to depend largely on the action of the pulvinus, for the petiole
at night was curved upwards at this point; and during the day, whilst
the petiole was horizontal, the lower surface of the pulvinus was
wrinkled with the upper surface tense. Although the cotyledons at an
advanced age do not rise at night to a higher inclination than whilst
young, yet they have to pass through a larger angle (in one instance
amounting to 63°) to gain their nocturnal position, as they are
generally deflected beneath the horizon during the day. Even with the
11-days’ old seedling the movement did not depend exclusively on the
pulvinus, for the blade where joined to the petiole was curved upwards,
and this must be attributed to unequal growth. Therefore the periodic
movements of the cotyledons of ‘O. corniculata’ depend on two distinct
but conjoint actions, namely, the expansion of the cells of the
pulvinus and on the growth of the upper part of the petiole, including
the base of the blade.
Lotus Jacoboeus.—The seedlings of this plant present a case parallel to
that of Oxalis corniculata in some respects, and in others unique, as
far as we have seen. The cotyledons during the first 4 or 5 days of
their life do not exhibit any plain nocturnal movement; but afterwards
they stand vertically or almost vertically up at night. There is,
however, some degree of variability in this respect, apparently
dependent on the season and on the degree to which they have been
illuminated during
the day. With older seedlings, having cotyledons 4 mm. in length, which
rise considerably at night, there is a well-developed pulvinus close to
the blade, colourless, and rather narrower than the rest of the
petiole, from which it is abruptly separated. It is formed of a mass of
small cells of an average length of .021 mm.; whereas the cells in the
lower part of the petiole are about .06 mm., and those in the blade
from .034 to .04 mm. in length. The epidermic cells in the lower part
of the petiole project conically, and thus differ in shape from those
over the pulvinus.
Turning now to very young seedlings, the cotyledons of which do not
rise at night and are only from 2 to 2½ mm. in length, their petioles
do not exhibit any defined zone of small cells, destitute of
chlorophyll and differing in shape exteriorly from the lower ones.
Nevertheless, the cells at the place where a pulvinus will afterwards
be developed are smaller (being on an average .015 mm. in length) than
those in the lower parts of the same petiole, which gradually become
larger in proceeding downwards, the largest being .030 mm. in length.
At this early age the cells of the blade are about .027 mm. in length.
We thus see that the pulvinus is formed by the cells in the uppermost
part of the petiole, continuing for only a short time to increase in
length, then being arrested in their growth, accompanied by the loss of
their chlorophyll grains; whilst the cells in the lower part of the
petiole continue for a long time to increase in length, those of the
epidermis becoming more conical. The singular fact of the cotyledons of
this plant not sleeping at first is therefore due to the pulvinus not
being developed at an early age.
We learn from these two cases of Lotus and Oxalis, that the development
of a pulvinus follows from the growth of the cells over a small defined
space of the petiole being almost arrested at an early age. With Lotus
Jacobæus the cells at first increase a little in length; in Oxalis
corniculata they decrease a little, owing to self-division. A mass of
such small cells forming a pulvinus, might therefore be either acquired
or lost without any special difficulty, by different species in the
same natural genus: and we know that
with seedlings of Trifolium, Lotus, and Oxalis some of the species have
a well-developed pulvinus, and others have none, or one in a
rudimentary condition. As the movements caused by the alternate
turgescence of the cells in the two halves of a pulvinus, must be
largely determined by the extensibility and subsequent contraction of
their walls, we can perhaps understand why a large number of small
cells will be more efficient than a small number of large cells
occupying the same space. As a pulvinus is formed by the arrestment of
the growth of its cells, movements dependent on their action may be
long-continued without any increase in length of the part thus
provided; and such long-continued movements seem to be one chief end
gained by the development of a pulvinus. Long-continued movement would
be impossible in any part, without an inordinate increase in its
length, if the turgescence of the cells was always followed by growth.
Disturbance of the Periodic Movements of Cotyledons by Light.—The
hypocotyls and cotyledons of most seedling plants are, as is well
known, extremely heliotropic; but cotyledons, besides being
heliotropic, are affected paratonically (to use Sachs’ expression) by
light; that is, their daily periodic movements are greatly and quickly
disturbed by changes in its intensity or by its absence. It is not that
they cease to circumnutate in darkness, for in all the many cases
observed by us they continued to do so; but the normal order of their
movements in relation to the alternations of day and night is much
disturbed or quite annulled. This holds good with species the
cotyledons of which rise or sink so much at night that they may be said
to sleep, as well as with others which rise only a little. But
different species are affected in very different degrees by changes in
the light.
For instance, the cotyledons of Beta vulgaris, Solanum lycopersicum,
Cerinthe major, and Lupinus luteus, when placed in darkness, moved down
during the afternoon and early night, instead of rising as they would
have done if they had been exposed to the light. All the individuals of
the Solanum did not behave in the same manner, for the cotyledons of
one circumnutated about the same spot between 2.30 and 10 P.M. The
cotyledons of a seedling of Oxalis corniculata, which was feebly
illuminated from above, moved downwards during the first morning in the
normal manner, but on the second morning it moved upwards. The
cotyledons of Lotus Jacoboeus were not affected by 4 h. of complete
darkness, but when placed under a double skylight and thus feebly
illuminated, they quite lost their periodical movements on the third
morning. On the other hand, the cotyledons of Cucurbita ovifera moved
in the normal manner during a whole day in darkness.
Seedlings of Githago segetum were feebly illuminated from above in the
morning before their cotyledons had expanded, and they remained closed
for the next 40 h. Other seedlings were placed in the dark after their
cotyledons had opened in the morning and these did not begin to close
until about 4 h. had elapsed. The cotyledons of Oxalis rosea sank
vertically downwards after being left for 1 h. 20 m. in darkness; but
those of some other species of Oxalis were not affected by several
hours of darkness. The cotyledons of several species of Cassia are
eminently susceptible to changes in the degree of light to which they
are exposed: thus seedlings of an unnamed S. Brazilian species (a large
and beautiful tree) were brought out of the hot-house and placed on a
table in the middle of a room with two north-east and one north-west
window, so that they were fairly well illuminated, though of course
less so than in the hot-house, the day being moderately bright; and
after 36 m. the cotyledons which had been horizontal rose up vertically
and closed together as when asleep; after thus remaining on the table
for 1 h. 13 m. they began to open. The cotyledons of young seedlings of
another Brazilian species and of C. neglecta, treated in the same
manner, behaved similarly, excepting that they did not rise up quite so
much: they again became horizontal after about an hour.
Here is a more interesting case: seedlings of Cassia tora in two pots,
which had stood for some time on the table in the room just described,
had their cotyledons horizontal. One pot was now exposed for 2 h. to
dull sunshine, and the cotyledons
remained horizontal; it was then brought back to the table, and after
50 m. the cotyledons had risen 68° above the horizon. The other pot was
placed during the same 2 h. behind a screen in the room, where the
light was very obscure, and the cotyledons rose 63° above the horizon;
the pot was then replaced on the table, and after 50 m. the cotyledons
had fallen 33°. These two pots with seedlings of the same age stood
close together, and were exposed to exactly the same amount of light,
yet the cotyledons in the one pot were rising, whilst those in the
other pot were at the same time sinking. This fact illustrates in a
striking manner that their movements are not governed by the actual
amount, but by a change in the intensity or degree of the light. A
similar experiment was tried with two sets of seedlings, both exposed
to a dull light, but different in degree, and the result was the same.
The movements of the cotyledons of this Cassia are, however, determined
(as in many other cases) largely by habit or inheritance, independently
of light; for seedlings which had been moderately illuminated during
the day, were kept all night and on the following morning in complete
darkness; yet the cotyledons were partially open in the morning and
remained open in the dark for about 6 h. The cotyledons in another pot,
similarly treated on another occasion, were open at 7 A.M. and remained
open in the dark for 4 h. 30 m., after which time they began to close.
Yet these same seedlings, when brought in the middle of the day from a
moderately bright into only a moderately dull light raised, as we have
seen, their cotyledons high above the horizon.
Sensitiveness of Cotyledons to contact.—This subject does not possess
much interest, as it is not known that sensitiveness of this kind is of
any service to seedling plants. We have observed cases in only four
genera, though we have vainly observed the cotyledons of many others.
The genus cassia seems to be pre-eminent in this respect: thus, the
cotyledons of C. tora, when extended horizontally, were both lightly
tapped with a very thin twig for 3 m. and in the course of a few
minutes they formed together an angle of 90°, so that each had risen
45°. A single cotyledon of another seedling was tapped in a like manner
for 1 m., and it rose 27° in 9 m.; and after eight additional minutes
it had risen 10° more; the opposite cotyledon, which was not tapped,
hardly moved at all. The cotyledons in all these cases became
horizontal again in less than half an hour. The pulvinus is the most
sensitive part, for on slightly pricking three cotyledons with a
pin in this part, they rose up vertically; but the blade was found also
to be sensitive, care having been taken that the pulvinus was not
touched. Drops of water placed quietly on these cotyledons produced no
effect, but an extremely fine stream of water, ejected from a syringe,
caused them to move upwards. When a pot of seedlings was rapidly hit
with a stick and thus jarred, the cotyledons rose slightly. When a
minute drop of nitric acid was placed on both pulvini of a seedling,
the cotyledons rose so quickly that they could easily be seen to move,
and almost immediately afterwards they began to fall; but the pulvini
had been killed and became brown.
The cotyledons of an unnamed species of Cassia (a large tree from S.
Brazil) rose 31° in the course of 26 m. after the pulvini and the
blades had both been rubbed during 1 m. with a twig; but when the blade
alone was similarly rubbed the cotyledons rose only 8°. The remarkably
long and narrow cotyledons, of a third unnamed species from S. Brazil,
did not move when their blades were rubbed on six occasions with a
pointed stick for 30 s. or for 1 m.; but when the pulvinus was rubbed
and slightly pricked with a pin, the cotyledons rose in the course of a
few minutes through an angle of 6°o. Several cotyledons of C. neglecta
(likewise from S. Brazil) rose in from 5 m. to 15 m. to various angles
between 16v and 34°, after being rubbed during 1 m. with a twig. Their
sensitiveness is retained to a somewhat advanced age, for the
cotyledons of a little plant of C. neglecta, 34 days old and bearing
three true leaves, rose when lightly pinched between the finger and
thumb. Some seedlings were exposed for 30 m. to a wind (temp. 50° F.)
sufficiently strong to keep the cotyledons vibrating, but this to our
surprise did not cause any movement. The cotyledons of four seedlings
of the Indian C. glauca were either rubbed with a thin twig for 2 m. or
were lightly pinched: one rose 34°; a second only 6°; a third 13°; and
a fourth 17°. A cotyledon of C. florida similarly treated rose 9°; one
of C. corymbosa rose 7½°, and one of the very distinct C. mimosoides
only 6°. Those of C. pubescens did not appear to be in the least
sensitive; nor were those of C. nodosa, but these latter are rather
thick and fleshy, and do not rise at night or go to sleep.
Smithia sensitiva.—This plant belongs to a distinct sub-order of the
Leguminosae from Cassia. Both cotyledons of an oldish seedling, with
the first true leaf partially unfolded, were rubbed for 1 m. with a
fine twig, and in 5 m. each rose 32°; they
remained in this position for 15 m., but when looked at again 40 m.
after the rubbing, each had fallen 14°. Both cotyledons of another and
younger seedling were lightly rubbed in the same manner for 1 m., and
after an interval of 32 m. each had risen 30°. They were hardly at all
sensitive to a fine jet of water. The cotyledons of S. Pfundii, an
African water plant, are thick and fleshy; they are not sensitive and
do not go to sleep.
Mimosa pudica and albida.—The blades of several cotyledons of both
these plants were rubbed or slightly scratched with a needle during 1
m. or 2 m.; but they did not move in the least. When, however, the
pulvini of six cotyledons of M. pudica were thus scratched, two of them
were slightly raised. In these two cases perhaps the pulvinus was
accidentally pricked, for on pricking the pulvinus of another cotyledon
it rose a little. It thus appears that the cotyledons of Mimosa are
less sensitive than those of the previously mentioned plants.[29]
[29] The sole notice which we have met with on the sensitiveness of
cotyledons, relates to Mimosa; for Aug. P. De Candolle says (‘Phys.
Vég.,’ 1832, tom. ii. p. 865), “les cotyledons du M. pudica tendent à
se raprocher par leurs faces supérieures lorsqu’on les irrite.”
Oxalis sensitiva.—The blades and pulvini of two cotyledons, standing
horizontally, were rubbed or rather tickled for 30 s. with a fine split
bristle, and in 10 m. each had risen 48°; when looked at again in 35 m.
after being rubbed they had risen 4° more; after 30 additional minutes
they were again horizontal. On hitting a pot rapidly with a stick for 1
m., the cotyledons of two seedlings were considerably raised in the
course of 11 m. A pot was carried a little distance on a tray and thus
jolted; and the cotyledons of four seedlings were all raised in 10 m.;
after 17 m. one had risen 56°, a second 45°, a third almost 90°, and a
fourth 90°. After an additional interval of 40 m. three of them had
re-expanded to a considerable extent. These observations were made
before we were aware at what an extraordinarily rapid rate the
cotyledons circumnutate, and are therefore liable to error.
Nevertheless it is extremely improbable that the cotyledons in the
eight cases given, should all have been rising at the time when they
were irritated. The cotyledons of Oxalis Valdiviana and rosea were
rubbed and did not exhibit any sensitiveness.
Finally, there seems to exist some relation between
the habit of cotyledons rising vertically at night or going to sleep,
and their sensitiveness, especially that of their pulvini, to a touch;
for all the above-named plants sleep at night. On the other hand, there
are many plants the cotyledons of which sleep, and are not in the least
sensitive. As the cotyledons of several species of Cassia are easily
affected both by slightly diminished light and by contact, we thought
that these two kinds of sensitiveness might be connected; but this is
not necessarily the case, for the cotyledons of Oxalis sensitiva did
not rise when kept on one occasion for 1½ h., and on a second occasion
for nearly 4 h., in a dark closet. Some other cotyledons, as those of
Githago segetum, are much affected by a feeble light, but do not move
when scratched by a needle. That with the same plant there is some
relation between the sensitiveness of its cotyledons and leaves seems
highly probable, for the above described Smithia and Oxalis have been
called sensitiva, owing to their leaves being sensitive; and though the
leaves of the several species of Cassia are not sensitive to a touch,
yet if a branch be shaken or syringed with water, they partially assume
their nocturnal dependent position. But the relation between the
sensitiveness to contact of the cotyledons and of the leaves of the
same plant is not very close, as may be inferred from the cotyledons of
Mimosa pudica being only slightly sensitive, whilst the leaves are well
known to be so in the highest degree. Again, the leaves of Neptunia
oleracea are very sensitive to a touch, whilst the cotyledons do not
appear to be so in any degree.