At the meeting of the Association of Nurserymen in Chicago, last July,
one of our prominent horticulturists described leaf variegation as a
disease. Incidentally this brought up the question: Does the graft
affect the stock upon which it is inserted?
Much confusion of ideas exists upon this subject, largely due to a loose
application of the term disease. Strictly speaking, this term is only
applicable to that which shows the health of the plant to be impaired.
It should be distinguished from aberrant or abnormal forms, for these
are not necessarily indicative of disease. Nobody thinks of saying that
red or striped roses are diseased because they are departures in color
from the white flower of the type species; or that white, yellow, or
striped roses are diseased when the color of the type species is red.
Nobody thinks of saying that double flowers are evidences of disease in
the plant, or that diminution in the size of leaves or variation in
their form is a disease. Why then should it be said that because leaves
may become of some other color than green, or become party-colored,
therefore they are diseased? If it be said that flowers are not leaves,
and that therefore the analogy is not a good one, the reply is, that
flowers in all their parts, and fruits also, are only leaves differently
developed from the type. This fact is a proven one, and so admitted to
be by all botanists and vegetable physiologists of the present day. If
it be objected that by becoming double, flowers lose the power of
reproducing the variety or species, the answer is, that this loss of
power is not necessarily the result of disease, but may arise from
various other causes. Because an animal is castrated, it surely will not
be claimed that therefore it is diseased. In man and in the higher
animals the power of reproduction ceases at certain ages, but it cannot
therefore be said that such men or animals are diseased. Neither is a
redundancy of parts an unequivocal evidence of disease.
Topknot fowls and ducks are as healthy as those which do not have such
appendages, and a Shetland pony is as healthy as a Percheron horse,
notwithstanding the difference in their size and weight. Again, color in
block or in variegation is not positive evidence of disease in animal
life. The white Caucasian is as healthy as the negro, the copper-colored
Malay as the red Indian. The horse, ox, and hog run through white and
red to black both in solid and party-color, and all are equally healthy;
so with the rabbit, dog, cat, and others of our domestic animals. In
wild animals, birds, reptiles, fishes, and insects, it is the same, so
that mere difference in color or combinations of color are not _prima
facie_ evidence of disease.
But some will say this may be true of animal life, but not of plant
life. That there is a strong and evident analogy, the one with the
other, is now universally admitted by physiologists. Formerly many
physiologists considered leaf variegation a disease, because it
generally ran in stripes lengthwise of the leaf or in spots. In the
former case it was supposed to originate from disease in the leaf cells
of the leaf stalk, which, as the cells grow longitudinally, naturally
prolonged it to the end of the leaf. But the originating of varieties in
which the variegation did not assume this form, with other
considerations, has done much to upset this theory. In the variegated
leaved snowberry we have the center and border of the leaf green,
separated the one from the other by an isolated white or yellow zone. In
the zebra-leaved eulalia and the zebra-leaved juncus, from Japan, we
have the variegation of the leaf transversely instead of longitudinally,
so that according to the old theory we have the anomaly of a healthy
portion of the leaf producing an unhealthy portion, and that again a
healthy one, and thus alternately along the whole length of the leaf.
When we dissect a leaf in its primal development, we find that its cells
contain colorless globules, by botanists called chlorophyl or
phyto-color; these undergo changes according as they are acted upon by
light, oxygen, or other agents, producing green, yellow, red, and other
tints. This chlorophyl only exists in the outer or superficial cells of
the parenchyma or cellular tissue of the leaf, and thus differs from
starch and other substances produced in the internal cells, from which
the light is more or less excluded. It is a fatty or wax-like substance,
readily dissolved in alcohol or ether. The primal color of all leaves
and flowers is white or a pale yellowish hue, as can readily be seen by
cutting open a leaf or flower bud. The seed leaves of the French bean
are white when they come out of the earth, but they become green an hour
afterward under the influence of bright sunshine. A case is on record
where in a certain section, some miles in extent, in this country, about
the time of the trees coming into leaf, the sun did not shine for twenty
days; the leaves developed to nearly their full size, but were of a pale
or whitish color; finally, one forenoon the sun shone out fully, and by
the middle of the afternoon the trees were in full summer dress. These
facts show that the green color of leaves is due to the action of light.
Variegation is sometimes produced independently of the chlorophyl, as in
_Begonia argyrostigma_ and _Carduus marianus_, in which it is produced
by a layer of air interposed between the epidermis or outer skin of the
leaf and the cells beneath; this gives the leaf a bright, silvery
appearance.
To what, then, are we to ascribe leaf variegation? I think that it is
entirely due to diminished root power; by this I do not mean that the
roots are diseased, but that they are either in an aberrant or abnormal
state; but disease cannot be predicated upon either of these states. To
explain: everybody knows _Spirea callosa_ to be a strong growing shrub,
having umbels of rosy-colored flowers and strong, stout roots; the white
flowered variety is quite dwarf, is more leafy and bushy than the
species, and has more fibrous and delicate roots than the type; the
crisp-leaved variety is still more dwarf, very bushy, and very leafy,
and has very fine threadlike roots. This would indicate that the
aberrance is in the roots; the two varieties are much more leafy in
proportion to their size than the species, so that if the leaves
controlled the roots, the latter should have been larger in proportion
than those of the species. Again, once when, in the autumn, I was
preparing my greenhouse plants for their winter quarters, I cut back a
"Lady Plymouth" geranium, which chanced to be set away in a cool and
somewhat damp cellar. When discovered the following February and started
into growth in the greenhouse it produced nothing but solid green
leaves, and never afterward produced a variegated leaf. This I
attributed to its having gained greater root power during its long
season of rest. By this I mean that the roots had grown and greatly
increased in size, although there had not been any leaf growth. That
roots under certain circumstances do so is well known. The roots of fir
trees have been found alive and growing forty five years after the
trunks were felled. The same has occurred in an ash tree after its trunk
had been sawn off level with the ground. A root of _Ipomea sellowii_ has
been known to keep on growing for twelve years after its top had been
destroyed by frost; and in all that time it never made buds or leaves,
yet it increased to seven times its original weight. The tuberous roots
of some of the _Tropoeolums_ will continue to grow and increase in
size after the tops have been accidentally broken off; and potatoes
buried so deep in the earth that they cannot produce tops will produce a
crop of new potatoes.
On the other hand, I have had an oak-leaved geranium overlooked in a
corner of the greenhouse until it was almost dried up for lack of water.
When its branches were pruned back and it was started into growth only
one branch showed the almost black center of the leaf, all the rest were
clear green. This was an evident case of diminished root power, but the
plant grew as thriftily as ever. The lack of the dark marking in the
leaves was equivalent to the variegation in other varieties, only in a
reverse direction.
In practice, when gardeners wish to produce an abnormal condition in a
tree or plant, they will, if they wish to dwarf it, graft it on a
species or variety of diminished root power, and contrariwise, if they
wish to increase its growth, will graft it upon a stock of strong root
power. But in neither case can the graft be said to be diseased by the
action of the roots of the stock.
When this root power is so far diminished as to produce complete
albinism, the shoots from such roots appear to partake of this
diminished power, and to lose the power of making roots, and thus become
very difficult to propagate. It is sometimes said that albino cuttings
cannot be rooted at all, but this is a mistake, for I have succeeded in
striking such cuttings from the variegated leaved _Hydrangea_. It
required much care to do it; they did not, however, retain their albino
character after they rooted and started into growth.
Albinism and white variegation in leaves appear to be due to the
chlorophyl in such leaves being able to resist the action of the three
(red, yellow, and blue) rays of light. What we call color in any
substance or thing is due to its reflecting these different rays in
various proportions of combination and absorbing the rest of them, the
various proportions giving the various shades of color. White is due to
the reflection of all of them, and black to the absorption of them. In
some plants with variegated foliage we have the curious fact that the
cells containing chlorophyl reflecting one color produce cells which
reflect an entirely different color. In the coleus "Lady Burrill," for
instance, the lower half of the leaf is of a deep violet-crimson color,
and the upper half is golden yellow. In other varieties of coleus, in
_Perilla nankiensis_, and other plants, we have foliage without a
particle of green in it, and yet they are perfectly healthy. This shows
that green leaves are not absolutely necessary to the health of a plant.
As a proof of leaf variegation being a disease, the speaker alluded to
cited a case in which a green leaved abutilon, upon which a variegated
leaved variety had been grafted, threw out a variegated leaved shoot
below the graft. This can easily be explained. The growth of the trunk
or stem of all exogenous plants, or those which increase in size on the
outside of the stem, is brought about by the descent of certain
formative tissue called cambium, elaborated by the leaves and descending
between the old wood and the bark, where it is formed into alburnum or
woody matter. Some think that it is also formed by the roots and ascends
from them as well as descending from the leaves. Be this as it may,
there is no doubt about its descent. In such comparatively soft-wooded,
free growing plants as the abutilon the descent of the cambium is very
free and in considerable quantity, so that the stock would soon be
inclosed in a layer of it descending from the graft. When being
converted into woody matter it also forms adventitious buds which under
certain favorable circumstances will emit shoots of the same character
as the graft from which it was derived. The graft is such cases may be
said to inclose the stock in a tube of its own substance, leaving the
stock unaffected otherwise. The variegated shoot in this case was in
reality derived from the downward growth of the graft and not from the
original stock, which was not therefore contaminated by the graft. In
cases where the stock is of much slower growth than the graft, or the
graft is inserted upon a stock of some other species, the descending
cambium does not inclose the stock, but makes layers of wood on the stem
of the graft, which thus, as is frequently seen, overgrows the stock,
sometimes to such an extent as to make it unsightly. Nobody ever saw an
apple shoot from a crab stock, a pear from a quince stock, or a peach
shoot from a plum stock. This is one of the arguments in favor of the
view that cambium also rises from the roots.
Again, to show that the stock is not affected by the graft, or the graft
by the stock, except as to root power, let any person graft a white beet
upon a red beet, or contrariwise, when about the size of a goosequill,
and when they have attained their full growth, by dividing the beet
lengthwise he will find the line of demarkation between the colors
perfectly distinct, neither of them running into the other.
The theory that leaf variegation is a disease has been held by many
distinguished botanists and is in nowise new. But this theory has been
controverted, and we think successfully, by other botanists, and it is
not now accepted by the more advanced vegetable physiologists. There are
now so many acute and industrious students and observers in every
department of science, and the accumulation of facts is so rapid and so
great, that very many of the older theories are being set aside as not
in accord with the newly discovered facts. A student brought up in
institutions where the old theories are inculcated has afterward to
spend half his time in unlearning what he had been previously taught,
and the other half in studying the new facts brought to his notice and
testing the theories promulgated by men of science. Botanical science
does not wholly consist in the classification and nomenclature of
plants, but largely consists in a knowledge of vegetable anatomy and
physiology, and these require much study and some knowledge of other
sciences, such as chemistry, meteorology, geology, etc. Without such
general knowledge it is difficult to form a harmonious theory in regard
to any of the phenomena of plant life.
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