18. Having obtained a general idea of the growth and parts of a
phanerogamous plant from the common Flax of the field, the seeds and
seedlings of other familiar plants may be taken up, and their variations
from the assumed pattern examined.
19. =Germinating Maples= are excellent to begin with, the parts being so
much larger than in Flax that a common magnifying glass, although
convenient, is hardly necessary. The only disadvantage is that fresh
seeds are not readily to be had at all seasons.
[Illustration: Fig. 11. Embryo of Sugar Maple, cut through lengthwise
and taken out of the seed. 12, 13. Whole embryo of same just beginning
to grow; _a_, the stemlet or caulicle, which in 13 has considerably
lengthened.]
20. The seeds of Sugar Maple ripen at the end of summer, and germinate
in early spring. The embryo fills the whole seed, in which it is nicely
packed; and the nature of the parts is obvious even before growth
begins. There is a stemlet (caulicle) and a pair of long and narrow
seed-leaves (cotyledons), doubled up and coiled, green even in the seed,
and in germination at once unfolding into the first pair of
foliage-leaves, though of shape quite unlike those that follow.
21. Red Maple seeds are ripe and ready to germinate at the beginning of
summer, and are therefore more convenient for study. The cotyledons are
crumpled in the seed, and not easy to straighten out until they unfold
themselves in germination. The story of their development into the
seedling is told by the accompanying Fig. 14-20; and that of Sugar Maple
is closely similar. No plumule or bud appears in the embryo of these two
Maples until the seed-leaves have nearly attained their full growth and
are acting as foliage-leaves, and until a root is formed below. There is
no great store of nourishment in these thin cotyledons; so further
growth has to wait until the root and seed-leaves have collected and
elaborated sufficient material for the formation of the second internode
and its pair of leaves, which lending their help the third pair is more
promptly produced, and so on.
22. Some change in the plan comes with the Silver or Soft White Maple.
(Fig. 21-25). This blossoms in earliest spring, and it drops its large
and ripened keys only a few weeks later. Its cotyledons have not at all
the appearance of leaves; they are short and broad, and (as there is no
room to be saved by folding) they are straight, except a small fold at
the top,--a vestige of the habit of Maples in general. Their unusual
thickness is due to the large store of nutritive matter they contain,
and this prevents their developing into actual leaves. Correspondingly,
their caulicle does not lengthen to elevate them above the surface of
the soil; the growth below the cotyledons is nearly all of root. It is
the little plumule or bud between them which makes the upward growth,
and which, being well fed by the cotyledons, rapidly develops the next
pair of leaves and raises them upon a long internode, and so on. The
cotyledons all the while remain below, in the husk of the fruit and
seed, and perish when they have yielded up the store of food which they
contained.
[Illustration: Fig. 14. One of the pair of keys or winged fruits of Red
Maple; the seed-bearing portion cut open to show the seed. 15. Seed
enlarged, and divided to show the crumpled embryo which fills it. 16.
Embryo taken out and partly opened. 17. Embryo which has unfolded in
early stage of germination and begun to grow. 18. Seedling with next
joint of stem and leaves apparent; and 19 with these parts full-grown,
and bud at apex for further growth. 20. Seedling with another joint of
stem and pair of leaves.]
23. So, even in plants so much alike as Maples, there is considerable
difference in the amount of food stored up in the cotyledons by which
the growth is to be made; and there are corresponding differences in the
germination. The larger the supply to draw upon, the stronger the
growth, and the quicker the formation of root below and of stem and
leaves above. This deposit of food thickens the cotyledons, and renders
them less and less leaf-like in proportion to its amount.
[Illustration: Fig. 21. Fruit (one key) of Silver Maple, Acer
dasycarpum, of natural size, the seed-bearing portion divided to show
the seed. 22. Embryo of the seed taken out. 23. Same opened out, to show
the thick cotyledons and the little plumule or bud between them. 24.
Germination of Silver Maple, natural size; merely the base of the fruit,
containing the seed, is shown. 25. Embryo of same, taken out of the
husk; upper part of growing stem cut off, for want of room.]
24. =Examples of Embryos with thickened Cotyledons.= In the Pumpkin and
Squash (Fig. 26, 27), the cotyledons are well supplied with nourishing
matter, as their sweet taste demonstrates. Still, they are flat and not
very thick. In germination this store is promptly utilized in the
development of the caulicle to twenty or thirty times its length in the
seed, and to corresponding thickness, in the formation of a cluster of
roots at its lower end, and the early production of the incipient
plumule; also in their own growth into efficient green leaves. The case
of our common Bean (Phaseolus vulgaris, Fig. 28-30) is nearly the same,
except that the cotyledons are much more gorged; so that, although
carried up into the air and light upon the lengthening caulicle, and
there acquiring a green color, they never expand into useful leaves.
Instead of this, they nourish into rapid growth the plumule, which is
plainly visible in the seed, as a pair of incipient leaves; and these
form the first actual foliage.
25. Very similar is the germination of the Beech (Fig. 31-33), except
that the caulicle lengthens less, hardly raising the cotyledons out of
the ground. Nothing would be gained by elevating them, as they never
grow out into efficient leaves; but the joint of stem belonging to the
plumule lengthens well, carrying up its pair of real foliage-leaves.
26. It is nearly the same in the Bean of the Old World (Vicia Faba, here
called Horse Bean and Windsor Bean): the caulicle lengthens very little,
does not undertake to elevate the heavy seed, which is left below or
upon the surface of the soil, the flat but thick cotyledons remaining
in it, and supplying food for the growth of the root below and the
plumule above. In its near relative, the Pea (Fig. 34, 35), this use of
cotyledons for storage only is most completely carried out. For they are
thickened to the utmost, even into hemispheres; the caulicle does not
lengthen at all; merely sends out roots from the lower end, and develops
its strong plumule from the upper, the seed remaining unmoved
underground. That is, in technical language, the germination is
_hypogaeous_.
[Illustration: Fig. 26. Embryo of Pumpkin-seed, partly opened. 27. Young
seedling of same.]
[Illustration: Fig. 28. Embryo of Common Bean (Phaseolus vulgaris):
caulicle bent down over edge of cotyledons. 29. Same germinating:
caulicle well lengthened and root beginning; thick cotyledons partly
spreading; and plumule (pair of leaves) growing between them. 30. Same,
older, with plumule developed into internode and pair of leaves.]
27. There is sufficient nourishment in the cotyledons of a pea to make a
very considerable growth before any actual foliage is required. So it is
the stem-portion of the plumule which is at first conspicuous and
strong-growing. Here, as seen in Fig. 35, its lower nodes bear each a
useless leaf-scale instead of an efficient leaf, and only the later ones
bear leaves fitted for foliage.
[Illustration: Fig. 31. A Beech-nut, cut across. 32. Beginning
germination of the Beech, showing the plumule growing before the
cotyledons have opened or the root has scarcely formed. 33. The same, a
little later, with the plumule-leaves developing, and elevated on a long
internode.]
[Illustration: Fig. 34. Embryo of Pea, i. e. a pea with the coats
removed; the short and thick caulicle presented to view. 35. Same in
advanced germination: the plumule has developed four or five internodes,
bearing single leaves; but the first and second leaves are mere scales,
the third begins to serve as foliage; the next more so.]
28. This _hypogaeous_ germination is exemplified on a larger scale by the
Oak (Fig. 36, 37) and Horse-chestnut (Fig. 38, 39); but in these the
downward growth is wholly a stout tap-root. It is not the caulicle; for
this lengthens hardly any. Indeed, the earliest growth which carries the
very short caulicle out of the shell comes from the formation of
foot-stalks to the cotyledons; above these develops the strong plumule,
below grows the stout root. The growth is at first entirely, for a long
time mainly, at the expense of the great store of food in the
cotyledons. These, after serving their purpose, decay and fall away.
[Illustration: Fig. 36. Half of an acorn, cut lengthwise, filled by the
very thick cotyledons, the base of which encloses the minute caulicle.
37. Oak-seedling.]
[Illustration: Fig. 38. Half of a horse-chestnut, similarly cut; the
caulicle is curved down on the side of one of the thick cotyledons. 39.
Horse-chestnut in germination; foot-stalks are formed to the cotyledons,
pushing out in their lengthening the growing parts.]
29. Such thick cotyledons never separate; indeed, they sometimes grow
together by some part of their contiguous faces; so that the germination
seems to proceed from a solid bulb-like mass. This is the case in a
horse-chestnut.
30. =Germinating Embryo supplied by its own Store of Nourishment=, i. e.
the store in the cotyledons. This is so in all the illustrations thus
far, essentially so even in the Flax. This nourishment was supplied by
the mother plant to the ovule and seed, and thence taken into the embryo
during its growth. Such embryos, filling the whole seed, are
comparatively large and strong, and vigorous in germination in
proportion to the amount of their growth while connected with the parent
plant.
31. =Germinating Embryo supplied from a Deposit outside of Itself.= This
is as common as the other mode; and it occurs in all degrees. Some
seeds have very little of this deposit, but a comparatively large
embryo, with its parts more or less developed and recognizable. In
others this deposit forms the main bulk of the seed, and the embryo is
small or minute, and comparatively rudimentary. The following
illustrations exemplify these various grades. When an embryo in a seed
is thus surrounded by a white substance, it was natural to liken the
latter to the white of an egg, and the embryo or germ to the yolk. So
the matter around or by the side of the embryo was called the _Albumen_,
i. e. the white of the seed. The analogy is not very good; and to avoid
ambiguity some botanists call it the ENDOSPERM. As that means in English
merely the inwards of a seed, the new name is little better than the old
one; and, since we do not change names in botany except when it cannot
be avoided, this name of _albumen_ is generally kept up. A seed with
such a deposit is _albuminous_, one with none is _exalbuminous_.
32. The ALBUMEN forms the main bulk of the seed in wheat, maize, rice,
buckwheat, and the like. It is the floury part of the seed. Also of the
cocoa-nut, of coffee (where it is dense and hard), etc.; while in peas,
beans, almonds, and in most edible nuts, the store of food, although
essentially the same in nature and in use, is in the embryo itself, and
therefore is not counted as anything to be separately named. In both
forms this concentrated food for the germinating plant is food also for
man and for animals.
[Illustration: Fig. 40. Seed of Morning Glory divided, moderately
magnified; shows a longitudinal section through the centre of the embryo
as it lies crumpled in the albumen. 41. Embryo taken out whole and
unfolded; the broad and very thin cotyledons notched at summit; the
caulicle below. 42. Early state of germination. 43. Same, more advanced;
caulicle or primary stem, cotyledons or seed-leaves, and below, the
root, well developed.]
33. For an albuminous seed with a well-developed embryo, the common
Morning Glory (Ipomoea purpurea, Fig. 40-43) is a convenient example,
being easy and prompt to grow, and having all the parts well apparent.
The seeds (duly soaked for examination) and the germination should be
compared with those of Sugar and Red Maple (19-21). The only essential
difference is that here the embryo is surrounded by and crumpled up in
the albumen. This substance, which is pulpy or mucilaginous in fresh and
young seeds, hardens as the seed ripens, but becomes again pulpy in
germination; and, as it liquefies, the thin cotyledons absorb it by
their whole surface. It supplements the nutritive matter contained in
the embryo. Both together form no large store, but sufficient for
establishing the seedling, with tiny root, stem, and pair of leaves for
initiating its independent growth; which in due time proceeds as in Fig.
44, 45.
[Illustration: Fig. 44. Seedling of Morning Glory more advanced (root
cut away); cotyledons well developed into foliage-leaves: succeeding
internode and leaf well developed, and the next forming. 45. Seedling
more advanced; reduced to much below natural size.]
34. Smaller embryos, less developed in the seed, are more dependent upon
the extraneous supply of food. The figures 46-53 illustrate four grades
in this respect. The smallest, that of the Peony, is still large enough
to be seen with a hand magnifying glass, and even its cotyledons may be
discerned by the aid of a simple stage microscope.
35. The broad cotyledons of Mirabilis, or Four-o'clock (Fig. 52, 53),
with the slender caulicle almost encircle and enclose the floury
albumen, instead of being enclosed in it, as in the other illustrations.
Evidently here the germinating embryo is principally fed by one of the
leaf-like cotyledons, the other being out of contact with the supply. In
the embryo of Abronia (Fig. 54, 55), a near relative of Mirabilis, there
is a singular modification; one cotyledon is almost wanting, being
reduced to a rudiment, leaving it for the other to do the work. This
leads to the question of the
36. =Number of Cotyledons.= In all the preceding illustrations, the
embryo, however different in shape and degree of development, is
evidently constructed upon one and the same plan, namely, that of two
leaves on a caulicle or initial stem,--a plan which is obvious even when
one cotyledon becomes very much smaller than the other, as in the rare
instance of Abronia (Fig. 54, 55). In other words, the embryos so far
examined are all
37. =Dicotyledonous=, that is, two-cotyledoned. Plants which are thus
similar in the plan of the embryo agree likewise in the general
structure of their stems, leaves, and blossoms; and thus form a class,
named from their embryo DICOTYLEDONES, or in English, DICOTYLEDONOUS
PLANTS. So long a name being inconvenient, it may be shortened into
DICOTYLS.
[Illustration: Fig. 46. Section of a seed of a Peony, showing a very
small embryo in the albumen, near one end. 47. This embryo detached, and
more magnified.]
[Illustration: Fig. 48. Section of a seed of Barberry, showing the
straight embryo in the middle of the albumen. 49. Its embryo detached.]
[Illustration: Fig. 50. Section of a Potato seed, showing the embryo
coiled in the albumen. 51. Its embryo detached.]
[Illustration: Fig. 52. Section of the seed of Mirabilis or
Four-o'clock, showing the embryo coiled round the outside of the
albumen. 53. Embryo detached; showing the very broad and leaf-like
cotyledons, applied face to face, and the pair incurved.]
[Illustration: Fig. 54. Embryo of Abronia umbellata; one of the
cotyledons very small. 55. Same straightened out.]
38. =Polycotyledonous= is a name employed for the less usual case in
which there are more than two cotyledons. The Pine is the most familiar
case. This occurs in all Pines, the number of cotyledons varying from
three to twelve; in Fig. 56, 57 they are six. Note that they are all on
the same level, that is, belong to the same node, so as to form a circle
or _whorl_ at the summit of the caulicle. When there are only three
cotyledons, they divide the space equally, are one third of the circle
apart. When only two they are 180 deg. apart, that is, are _opposite_.
39. The case of three or more cotyledons, which is constant in Pines and
in some of their relatives (but not in all of them), is occasional among
Dicotyls. And the polycotyledonous is only a variation of the
dicotyledonous type,--a difference in the number of leaves in the whorl;
for a pair is a whorl reduced to two members. Some suppose that there
are really only two cotyledons even in a Pine embryo, but these divided
or split up congenitally so as to imitate a greater number. But as
leaves are often in whorls on ordinary stems, they may be so at the very
beginning.
[Illustration: Fig. 56. Section of a Pine-seed, showing its
polycotyledonous embryo in the centre of the albumen, moderately
magnified. 57. Seedling of same, showing the freshly expanded six
cotyledons in a whorl, and the plumule just appearing.]
40. =Monocotyledonous= (meaning with single cotyledon) is the name of
the one-cotyledoned sort of embryo. This goes along with peculiarities
in stem, leaves, and flowers, which all together associate such plants
into a great class, called MONOCOTYLEDONOUS PLANTS, or, for shortness,
MONOCOTYLS. It means merely that the leaves are alternate from the very
first.
[Illustration: Fig. 58. Section of a seed of the Iris, or
Flower-de-Luce, enlarged, showing its small embryo in the albumen, near
the bottom. 59. A germinating seedling of the same, its plumule
developed into the first four leaves (alternate), the first one
rudimentary, the cotyledon remains in the seed.]
41. In Iris (Fig. 58, 59) the embryo in the seed is a small cylinder at
one end of the mass of the albumen, with no apparent distinction of
parts. The end which almost touches the seed coat is caulicle, the other
end belongs to the solitary cotyledon. In germination the whole
lengthens (but mainly the cotyledon) only enough to push the proximate
end fairly out of the seed; from this end the root is formed, and from a
little higher the plumule later emerges. It would appear therefore that
the cotyledon answers to a minute leaf rolled up, and that a chink
through which the plumule grows out is a part of the inrolled edges. The
embryo of Indian Corn shows these parts on a larger scale and in a more
open state (Fig. 66-68). There, in the seed, the cotyledon remains,
imbibing nourishment from the softened albumen, and transmitting it to
the growing root below and new-forming leaves above.
[Illustration: Fig. 60. Section of an Onion seed showing the slender and
coiled embryo in the albumen, moderately magnified. 61. Seed of same in
early germination.]
[Illustration: Fig. 62. Germinating Onion, more advanced, the chink at
base of cotyledon opening for the protrusion of the plumule, consisting
of a thread-shaped leaf. 63. Section of base of Fig. 62, showing plumule
enclosed. 64. Section of same later, plumule emerging. 65. Later stage
of 62, upper part cut off. 66. A grain of Indian Corn, flatwise, cut
away a little, so as to show the embryo, lying on the albumen which
makes the principal bulk of the seed. 67. A grain cut through the middle
in the opposite direction, dividing the embryo through its thick
cotyledon and its plumule, the latter consisting of two leaves, one
enclosing the other. 68. The embryo taken out whole; the thick mass is
the cotyledon, the narrow body partly enclosed by it is the plumule, the
little projection at its base is the very short radicle enclosed in the
sheathing base of the first leaf of the plumule.]
[Illustration: Fig. 69. Grain of Indian Corn in germination, the
ascending sprout is the first leaf of the plumule, enclosing the younger
leaves within, at its base the primary root has broken through. 70. The
same, advanced; the second and third leaves developing, while the
sheathing first leaf does not further develop.]
42. The general plan is the same in the Onion (Fig. 60-65), but with a
striking difference. The embryo is long, and coiled in the albumen of
the seed. To ordinary examination it shows no distinction of parts. But
germination plainly shows that all except the lower end of it is
cotyledon. For after it has lengthened into a long thread, the chink
from which the plumule in time emerges is seen at the base, or near it,
so the caulicle is extremely short, and does not elongate, but sends out
from its base a simple root, and afterwards others in a cluster. Not
only does the cotyledon lengthen enormously in the seedling, but (unlike
that of Iris, Indian Corn, and all the cereal grains) it raises the
comparatively light seed into the air, the tip still remaining in the
seed and feeding upon the albumen. When this food is exhausted and the
seedling is well established in the soil, the upper end decays and the
emptied husk of the seed falls away.
43. In Maize or Indian Corn (Fig. 66-70), the embryo is more developed
in the seed, and its parts can be made out. It lies against the starchy
albumen, but is not enclosed therein. The larger part of it is the
cotyledon, thickish, its edges involute, and its back in contact with
the albumen; partly enclosed by it is the well-developed plumule or bud
which is to grow. For the cotyledon remains in the seed to fulfil its
office of imbibing nourishment from the softened albumen, which it
conveys to the growing sprout; the part of this sprout which is visible
is the first leaf of the plumule rolled up into a sheath and enclosing
the rudiments of the succeeding leaves, at the base enclosing even the
minute caulicle. In germination the first leaf of the plumule develops
only as a sort of sheath, protecting the tender parts within; the second
and the third form the first foliage. The caulicle never lengthens: the
first root, which is formed at its lower end, or from any part of it,
has to break through the enclosing sheath; and succeeding roots soon
spring from all or any of the nodes of the plumule.
44. =Simple-stemmed Plants= are thus built up, by the continuous
production of one leaf-bearing portion of stem from the summit of the
preceding one, beginning with the initial stem (or caulicle) in the
embryo. Some Dicotyls and many Monocotyls develop only in this single
line of growth (as to parts above ground) until the flowering state is
approached. For some examples, see Cycas (Fig. 71, front, at the left);
a tall Yucca or Spanish Bayonet, and two Cocoa-nut Palms behind; at the
right, a group of Sugar-canes, and a Banana behind.
[Illustration: Fig. 71. Simple-stemmed vegetation.]