Asa Gray.

The botanical text-book, an introduction to scientific botany, both structural and systematic. For colleges, schools, and private students online

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by an obtusely conical mass of older cells, consisting of the super-
ficial tissue of the end of the radicle, pushed forward by the cell-
multiplication that commenced behind it, as already mentioned
(Fig. 108). As the original cells of this apex wear away or per-
ish, they are replaced by the layer beneath ; and so the advancing
point of the root consists, as inspection plainly shows, of older and




108



FIG. 107. A germinating embryo of Sugar Maple, still more advanced : a, the radicle elon-
gated into the first joint of stem, bearing the unfolded cotyledons or seed-leaves, b, and between
them the rudiments of the next pair of leaves; while from its lower extremity the root, r, is
formed.

FIG. 103. The lower end of the same root, magnified: a, the place where growth, through
the multiplication of cells by division, is principally taking place : b, the original apex of the
radicle, which has been carried onward by the growth that has taken place just behind it.



THE ROOT.



denser tissue than that behind it.* The point of every branch of
the root is capped in the same way. It follows that the so-called
spongioles or spongelets of the roots have no existence. Not only
are there no such special organs as are commonly spoken of, but
absorption evidently does not take place, to any considerable ex-
tent, through the older tissue of the point itself.

120. As to absorption by roots, the inspection of the root of a
germinating plantlet, or of any growing rootlet, even under a low
magnifying power, shows that they must imbibe the moisture that
bathes them, by endosmosis (37), through the whole recently formed
surface, and especially by the hair-like prolongations of the exterior
layer of cells, or fibrils, as they may be termed, which are copiously
borne by all young roots (Fig. 108). Fig. 109, 110, show some
of these root-hairs, and the tissue that bears them, more magni-
fied. These capillary tubes, of
great tenuity and with extremely
delicate walls, immensely increase
the surface which the rootlet ex-
poses, and play a more important
part in absorption than is gener-
ally supposed ; for they appear
to have attracted little attention.
These fibrils perish when the
growing season is over, or when
the root gets a little older ; at the
same time, the external layer of
cells that bears them, at first un-
distinguishable from the parenchy-
ma beneath, except perhaps in the size of the cells, hardens and
thickens into a sort of epidermis, or firmer skin, so as to arrest or
greatly restrain the imbibition. This epidermis (69) of the root
consists of less compressed cells than in parts exposed to the light,
and is destitute of stomates or breathing-pores (70).

121. The growth of the root and its branches keeps pace with the
development of the stem. As the latter shoots upward and expands
its leaves, from which water is copiously exhaled during vigorous




* It is a similar tissue that exfoliates from the point of some aquatic (as in
Lemna, Fig. 96), and many aerial roots (as in Pandanus), in the form of a
loose cup or sheath.



THE PRIMARY ROOT. 83'

vegetation, the former grow onward and continually renew the ten-
der, hygrometric tissue through which the absorption, required to
restore what is lost by evaporation or consumed in growth, is
principally effected. Hence the danger of disturbing the active
roots during the season of growth. In early summer, when the sap
is rapidly consumed by the fresh leaves, the rootlets are also in rap-
id action. The growth of the branches and roots being simultane-
ous, while new branchlets and leaves are developing, the rootlets are
extending at a corresponding rate, and their tender absorbing points
are most frequently renewed. They cannot now be removed from
the soil without destroying them, at the very time when their action
is essential to restore the liquid which is exhaled from the leaves.
But towards the close of summer, as the leaves grow languid and
the growth of the season is attained, the rootlets also cease to
grow, the loose tissue of their extremities, not being renewed, grad-
ually solidifies, and absorption at length ceases. This indicates
the proper period for transplanting, namely, in the autumn a'fter
vegetation is suspended, or in early spring before it recommences.

122. This elongation of roots by their advancing points alone is
admirably adapted to the conditions in which they are placed.
Growing as they do in a medium of such unequal resistance as the
soil, if roots increased like growing stems, by the elongation of the
whole body, they would be thrown, whenever the elongating force
was insufficient to overcome the resistance, into knotted or con-
torted shapes, very ill adapted for the free transmission of fluid.
But, lengthening only at their farthest extremity, they insinuate
themselves with great facility into the crevices or yielding parts of
the soil, and afterwards by their expansion in diameter enlarge the
cavity ; or, when arrested by insuperable obstacles, their advan-
cing points follow the surface of the opposing body until they reach
a softer medium. In this manner, too, they readily extend from
place to place, as the nourishment in their immediate vicinity is
consumed. Hence, also, may be derived a simple explanation of
the fact, that roots extend most rapidly and widely in the direction
of the most favorable soil, without supposing any prescience on the
part of the vegetable, as some have imagined.

123. The advancing extremity of the root consists of parenchy-
ma alone ; but bundles of vessels and woody tissue appear in the
forming root, soon after their appearance in the primordial stem
above : these form a central woody or fibrous portion, which con-



84 THE ROOT.

tinues to descend (by the transformation of a portion of the nas-
cent tissue) as the growing apex advances ; sometimes, although
not usually, inclosing a distinct pith, as the wood of the stem
does. The surrounding parenchymatous portion becomes the bark
of the root. Increase in diameter takes place in the same way as
in the stem. (Chap. IV. Sect. IV.)

124. We have taken the root of the seedling as an example and
epitome of that of the whole herb or tree ; as we rightly may ; for
in its whole development the root produces no other parts ; it
bears nothing but naked branches, which spring from different
portions of the surface of the main root, nearly as this sprung from
the radicle, and exactly imitate its growth. They and their rami-
fications are mere repetitions of the original descending axis, serv-
ing to multiply the amount of absorbing surface. The branches of
the root, moreover, shoot forth without apparent order ; or at least
in no order like that of the branches of the stem, which have a
symmetrical arrangement, dependent, as we shall see, upon the
arrangement of the leaves.

125. To the general statement that roots give birth to no other
organs, there is this abnormal, but by no means unusual exception,
that of producing buds and therefore sending up leafy branches.
Although not naturally furnished with buds, like the stem, yet,
under certain circumstances, those of many trees and shrubs, and
of several herbs, have the power of producing them abundantly.
Thus, when the trunk of a young Apple-tree or Poplar is cut off
near the ground, while the roots are vigorous and full of sap,
those which spread just beneath the surface produce buds, and
give rise to a multitude of young shoots. The roots of the Ma-
clura, or Osage Orange, habitually give rise to buds and branches.
Such buds are said to be irregular, or adventitious. This power,
however, roots share with every part of the vegetable that abounds
with parenchyma : even leaves are known to produce adventitious
buds.

. 126. The root has been illustrated from the highest class of
PhaBnogamous plants; in which the original root, or downward
prolongation of the axis, continues to grow, at least for a consider-
able time, and becomes a tap-root, or main trunk, from which
branches of larger or smaller size emanate. Often, however, this
main root early perishes or ceases to grow, and the branches take
its place. In some plants of the highest class (in the Gourd Fam-



ANNUAL, BIENNIAL, AND PERENNIAL ROOTS. 85

ily, for example), and in nearly the whole great class to which
Grasses, Lilies, and Palms belong, there is no one main trunk or
primary root from which the rest proceed ; but several roots spring
forth almost simultaneously from the radicle in germination, and
form a cluster of fibres, of nearly equal size (Fig. 111). Such
plants scarcely exhibit that distinct opposition of growth in the first
instance, already mentioned as one characteristic of PhaBnogamous
vegetation. Most Phsenogamous plants likewise shoot forth secon-
dary roots from the stem itself, the only kind produced by Cryp-
togamous plants. To these we must revert, after having consid-
ered some diversities connected with the duration and form of
roots, and an important subsidiary purpose which they often sub-
serve.

127. Annual Roots are those of a plant which springs from the
seed, flowers, and dies the same year or season. Such plants al-
ways have fibrous roots, composed of numerous slender branches,
fibres, or rootlets, proceeding laterally from the main or taproot,
which is very little enlarged, as in Mustard, &c. ; or else the whole
root divides at once into such fibrous branches, as in Barley (Fig.
Ill) and all annual Grasses. These multiplied rootlets are well
adapted for absorption from the soil, but for that alone. The food
which the roots of such a plant absorbs, after being digested and
elaborated in its leaves, is all expended in the production of new
leafy branches, and at length of flowers. The flowering process
and the maturing of the fruit exhaust the vegetable greatly (in a
manner hereafter to be explained), consuming all the nourishing
material which it contains, or storing it up in the fruit or seed for
its offspring ; and having no stock accumulated in the root or else-
where to sustain this draught, the plant perishes at the close of the
season, or whenever it has fully gone to seed.

128. Biennial Roots are those of plants which do not blossom
until the second season, after which they perish like annuals. In
these the root serves as a reservoir of nourishing, assimilated mat-
ter (27, 79) ; its cells therefore become gorged with starch (81),
vegetable jelly (83), sugar (84), &c. Such thickened roots are
said to be fleshy , and receive different names according to the
shapes they assume. When the accumulation takes place in the
main trunk or tap-root, it becomes conical^ as in the Carrot, Fig.
1 12, when it tapers regularly from the base or crown to the apex ;
it is fusiform or spindle-shaped when it tapers upwards as well as

8



86



THE ROOT.



downwards, as in the Radish, Fig. 113; or napiform or turnip-
shaped^ when much swollen at the base, so as to become broader
than long. If some of the branches or fibres are thickened, instead
of the main axis, the root is said to be fasciculated or clustered,
as in Fig. 114 ; or tuberiferous or tuberous, when they assume the
form of rounded knobs, as in Fig. 115; or palmate, when the
knobs are branched, as in Fig. 116. These must not be con-
founded with tubers, such as potatoes, which are forms of stems.
Most of these are biennial. Such plants (of which the Radish,
Carrot, Beet, and Turnip, among our esculents, are familiar exam-
ples) neither flower the first season, nor even expend in the pro-
duction of stems and branches much of the nourishment they gen-
erate ; but, forming a large tuft of leaves at the very surface of
the ground, they accumulate in the root nearly the whole sum-
mer's supply of nourishment. When vegetation is resumed the
following spring, they make a strong and rapid growth, shooting
forth a large stem, and bearing flowers, fruit, and seed, almost
wholly at the expense of the accumulation of the previous year ;
this store is soon consumed, therefore; and the plant, meanwhile
neglecting to form new roots, perishes from exhaustion.




129. Perennial Roots, A third class of herbs, and all woody
plants, do not so absolutely depend upon the stock of the previous
season, but annually produce new roots and form new accumula-
tions ; sometimes in separate portions of the root, as in the Dahlia

FIG. 111-116. Different kinds of roots.



SECONDARY ROOTS. 87

or the Orchis (Fig. 115), where, while one or more of such reser-
voirs is exhausted each year, others are providently formed for
the next year's sustenance ; and so on from year to year ; a por-
tion annually perishing, but the individual plant surviving indefi-
nitely. More commonly, the whole body and main branches of
the root are somewhat thickened ; or portions of the stem may
subserve this purpose, as in all tuberous herbs ; or the nourishing
matter may be widely distributed through the trunk, as in shrubs
and tree$. These are some of the modifications in this respect of
perennial plants, which survive, or at least their roots, and blossom
from year to year indefinitely.

130. Secondary Roots, (Also called Adventitious Roots.) Thus
far, the primitive root, that which originated from the base of the
embryo in germination, with its ramifications, has alone been con-
sidered. But roots habitually spring from any part of a growing
stem that lies on the ground, or is buried beneath its surface, so as
to provide the moisture and darkness they require ; for such roots
obey the ordinary tendency of the organ, avoiding the light, and
seeking to bury themselves in the soil. Most creeping plants pro-
duce them at every joint ; and most branches, when bent to the
ground and covered with earth, will strike root. So, often, will
separate pieces of young stems, if due care be taken ; as when
plants are propagated by cuttings. Stems commonly do not strike
root, except when in contact with the ground. To this, however,
there are various exceptions ; as in the case of

131. Aerial Roots, Some woody vines climb by such rootlets ;
as the Ivy, our own Poison Ivy (Rhus Toxicodendron), and the
Bignonia or Trumpet-Creeper, which in this way reach the sum-
mit of high trees. Such plants derive their nourishment from their
ordinary roots imbedded in the soil ; their copious aerial rootlets
merely serving for. mechanical support. Other plants produce
true aerial roots, which, emitted from the stem in the open air,
descend to the ground and establish themselves in the soil. This
may be observed, on a small scale, in the stems of Indian Corn,
where the lower joints often produce roots which grow to the
length of several inches before they reach the soil. More striking
cases of the kind abound in those tropical regions where the sultry
air, saturated with moisture for a large part of the year, favors the
utmost luxuriance of vegetation. The Pandanus or Screw-Pine (a
Palm-like tree, often cultivated in our conservatories) affords a



88



THE ROOT.



welt-known instance. The strong roots, emitted in the open air
from the lower part of the trunk, soon reach the soil, as is shown

in Fig. 117, giving
the tree the appear-
ance of having been
partially raised out
of the ground. The
famous Banyan-tree
(Fig. 119) affords a
still more striking
illustration. Here
the aerial rootlets
strike from the hor-
izontal branches of
the tree, often at a
great height, and
swing free in the
air, like pendent
cords ; but they fi-
nally reach and es-
tablish themselves
in the ground, where
they increase in di-
ameter and form numerous accessory trunks, surrounding the





FIG. 117. The Pandanua, or Screw-Pine ; with, 113, a Mangrove-tree (Rhizophora Mangle).
FIG. 119. The Banyan-tree, or Indian Fig (Ficus Indica).



EPIPHYTES OR AIR-PLANTS.



89



original bole and supporting the wide-spread canopy of branches
and foliage. Very similar is the economy of the Mangrove (Fig.
118), which inhabits muddy sea-shores throughout the tropics, and
even occurs sparingly on the coast of Florida and Louisiana. Its
aerial roots spring both from the main trunk, as in the Pandanus,
and from the branchlets, as in the Banyan. Moreover, this ten-
dency to shoot in the air is shown even in the embryo, which be-
gins to germinate while the pod is yet attached to the parent
branch ; the radicle, or root-end of the embryo, elongating into a
slender thread, which often reaches the ground from the height of
many yards, before the pod is detached. In this manner the Man-
grove forms those impenetrable maritime thickets which abound
on low, muddy shores, within the tropics.




132. Epiphytes, or Air-plants, exhibit a further peculiarity. Their
roots not only strike in the free air, but throughout their life have
no connection with the soil. They generally grow upon the trunks

FIG. 120. Oncidium Papilio, and, 121, Comparettia rosea; two epiphytes of the Orchis
Family ; showing the mode in which these Air-plants grow.

8*



90 THE ROOT.

and branches of trees ; their roots merely adhering to the bark to
fix the plant in its position, or else hanging loose in the air, from
which such plants draw all their nourishment. Of this kind are a
large portion of the gorgeous Orchidaceous plants of very warm
and humid climes, which are so much prized in hot-houses, and
which, in their flowers as well as their general aspect, exhibit such
fantastic and infinitely varied forms. Some of the flowers resem-
ble butterflies, or strange insects, in shape as well as in gaudy col-
oring ; such, for example, as the Oncidium Papilio (Fig. 1'20),
which we have selected for one of our illustrations. To another
family of Epiphytic plants belongs the Tillandsia, or Long Moss,
which, pendent in long and gray tangled clusters or festoons from
the branches of the Live-Oak or Long-leaved Pine, gives such a
peculiar and sombre aspect to the forests of the warmer portions
of our Southern States. They are called Air-plants, in allusion to
the source of their nourishment ; and Epiphytes, from their grow-
ing upon other plants, and in contradistinction to

133. Parasites, that not only grow upon other vegetables, but live
at their expense ; which Epiphytes do not. Parasitic plants may
be divided into two sorts, viz. : 1st, those that have green foli-
age, and 2d, those that are destitute of green foliage. They may
vary also in the degree of parasitism ; the greater number being
absolutely dependent upon the foster plant for nourishment, while
a few, such as the Curbed Fig (Clusia rosea) of tropical America,
often take root in the soil, and thence derive a part, or sometimes
the whole, of their support. This occurs only in

134. Green Parasites, or those furnished with green foliage, or
proper digestive organs of their own. These strike their roots
through the bark and directly into the new wood of the foster
plant ; whence they can draw little except the ascending, mostly
crude sap (79), which they have to assimilate in their own green
leaves. The Mistletoe is the most familiar exam-pie of this class.
It is always completely parasitic, being at no period connected
with the earth ; but the seed germinates upon the trunk or branch
of the tree where it happens to fall, and its nascent root, or rather
the woody mass that it produces in place of the root, penetrates
the bark of the foster stem, and forms as close a junction, ap-
parently, with its young wood as that of a natural branch. Some
species of Mistletoe, or of the same family, however, display no
proper green foliage, but are of a yellow or brown hue. On the



PARASITIC PLANTS. 91

other hand, imperfect root-parasites with green foliage have re-
cently been detected in more than one tribe of plants ; * thus ex-
hibiting intermediate states between the Green and the

135. Pale or Colored Parasites, that is, of other colors than
green ; such as Beech-drops, Orobanche, &c. These strike their
roots, or sucker-shaped discs, into the bark, mostly that of the root,
of other plants, and thence draw
their food from the sap already elab-
orated (79). They have according-
ly no occasion for digestive organs
of their own, and are in fact always
destitute of green foliage. In some
cases of the kind, as in the Dodder
(Fig. 122- 124), the seeds germinate
in the earth, from which the primi-
tive root derives its nourishment in
the ordinary manner ; but when the
slender twining stem reaches the
surrounding herbage, it gives out
aerial roots, which attach themselves
firmly to the surface of the support-
ing plant, penetrate its epidermis,
and feed upon its juices ; while the
original root and base of the stem perish, and the plant has no
longer any connection with the soil. Thus stealing its nourish-
ment ready prepared, it requires no proper digestive organs of its

* In England a Thesium was discovered by Mr. Mitten to attach its roots
parasitically, by suckers, to the roots of adjacent herbs. (It would be inter-
esting to know if this is the case with our Comandra.) Then Decaisne, recol-
lecting that Rhinanthaceous plants generally, all of which blacken more or
less in drying, were known to be uncultivable, and have the reputation, in
France and elsewhere, of being injurious to cereal and other plants in their
vicinity, was led to the discovery that plants of Rhinanthus, Melampyrum,
and of the allied genera, attached themselves by numerous suckers on their
roots to the roots of Grasses, shrubby plants, and even of trees, among which
they grow. Our handsome species of Gerardia are equally uncultivable,
doubtless on account of this partial parasitism.

FIG. 122. The common Dodder of the Northern States (Cuscuta Gronovii), of the natural
size, parasitic upon the stem of an herb: the uncoiled portion at the lower end shows the mode
of its attachment. 123. The coiled embryo taken from the seed, moderately magnified. 124. The
same in germination; the lower end elongating into a root; the upper into a thread-like leaf-
less slem.




92 THE ROOT.

own, and, consequently, does not produce leaves. This economy
is, as it were, foreshadowed in the embryo of the Dodder, which
is a slender thread spirally coiled in the seed (Fig. 123, 124), and
which presents no vestige of cotyledons or seed-leaves. A spe-
cies of Dodder infests and greatly injures flax in Europe, and
sometimes makes its appearance in our own flax-fields, having
been introduced with the imported seed. Some species make
great havoc in the clover- fields of the Old World.

136. Such parasites do not live upon all plants indiscriminately,
but only upon those whose elaborated juices furnish a propitious
nourishment. Some of them are restricted, or nearly so, to a par-
ticular species ; others show little preference, or are found indif-
ferently upon several species of different families. Their seeds,
in some cases, it is said, will germinate only when in contact with
the stem or root of .the species upon which they are destined to
live. Having no need of foliage, such plants may be reduced to a
stalk with a single flower or cluster of flowers, as in the different
kinds of Beech-drops,* the Cytinus, which is parasitic on the Cistus
of the South of Europe, &c. They may even be reduced to a
single flower directly parasitic on the bark of the foster plant,
without the intervention of any manifest stem. A truly wonderful
instance of this kind is furnished by that vegetable Titan, the Raf-
flesia Arnoldi of Sumatra (Fig. 125). The flower which was first




discovered grew upon the stem of a kind of grape-vine ; it meas-
ured nine feet in circumference, and weighed fifteen pounds ! Its
color is light orange, mottled with yellowish-white.

* See family Orolanchacece, in the second part of this work.

FIG. 125. Rafflesia Arnoldi ; an expanded flower, and a bud, directly parasitic on the stem
of a vine : reduced to the scale of half an inch to a foot.



THE STEM. 93

137. Among Cryptogamous plants, numerous Fungi are para-
sitic upon living, especially upon languishing vegetables ; others
infest living animals ; the rest feed on dead or decaying vegeta-
ble or animal matters : all are destitute of chlorophyll (87), or any
thing like green foliage. It is not improbable that our Monotropa,
or Indian Pipe, a pallid and fungus-like Pha3nogamous plant, draws
its nourishment, at least in great part, from the decaying leaves
among which it grows.



CHAPTER IV.

OR ASCENDING AXIS.

SECT. I. ITS GENERAL CHARACTERISTICS AND MODE OF GROWTH.

138. BESIDES the direction of its growth, the descending axis or
root we have found to be characterized by producing nothing ex-



Online LibraryAsa GrayThe botanical text-book, an introduction to scientific botany, both structural and systematic. For colleges, schools, and private students → online text (page 8 of 47)