65. It is a property of stems to produce roots. Stems do not spring from
roots in ordinary cases, as is generally thought, but roots from stems.
When perennial herbs arise from the ground, as they do at spring-time,
they rise from subterranean stems.
66. =The Primary Root= is a downward growth from the root-end of the
caulicle, that is, of the initial stem of the embryo (Fig. 5-7, 81). If
it goes on to grow it makes a _main_ or _tap-root_, as in Fig. 37, etc.
Some plants keep this main root throughout their whole life, and send
off only small side branches; as in the Carrot and Radish: and in
various trees, like the Oak, it takes the lead of the side-branches for
several years, unless accidentally injured, as a strong tap-root. But
commonly the main root divides off very soon, and is lost in the
branches. _Multiple primary roots_ now and then occur, as in the
seedling of Pumpkin (Fig. 27), where a cluster is formed even at the
first, from the root-end of the caulicle.
[Illustration: Fig. 81. Seedling Maple, of the natural size; the root
well supplied with root hairs, here large enough to be seen by the naked
eye. 82. Lower end of this root, magnified, the root seen just as
root-hairs are beginning to form a little behind the tip.]
67. =Secondary Roots= are those which arise from other parts of the
stem. Any part of the stem may produce them, but they most readily come
from the nodes. As a general rule they naturally spring, or may be made
to spring, from almost any young stem, when placed in favorable
circumstances,--that is, when placed in the soil, or otherwise supplied
with moisture and screened from the light. For the special tendency of
the root is to avoid the light, seek moisture, and therefore to bury
itself in the soil. _Propagation by division_, which is so common and so
very important in cultivation, depends upon the proclivity of stems to
strike root. Stems or branches which remain under ground give out roots
as freely as roots themselves give off branches. Stems which creep on
the ground most commonly root at the joints; so will most branches when
bent to the ground, as in propagation by _layering_; and propagation by
_cuttings_ equally depends upon the tendency of the cut end of a shoot
to produce roots. Thus, a piece of a plant which has stem and leaves,
either developed or in the bud, may be made to produce roots, and so
become an independent plant.
68. =Contrast between Stem and Root.= Stems are ascending axes; roots
are descending axes. Stems grow by the successive development of
internodes (13), one after another, each leaf-bearing at its summit (or
node); so that it is of the essential nature of a stem to bear leaves.
Roots bear no leaves, are not distinguishable into nodes and internodes,
but grow on continuously from the lower end. They commonly branch
freely, but not from any fixed points nor in definite order.
69. Although roots generally do not give rise to stems, and therefore do
not propagate the plant, exceptions are not uncommon. For as stems may
produce adventitious buds, so also may roots. The roots of the Sweet
Potato among herbs, and of the Osage Orange among trees freely produce
adventitious buds, developing into leafy shoots; and so these plants are
propagated by _root-cuttings_. But most growths of subterranean origin
which pass for roots are forms of stems, the common Potato for example.
70. Roots of ordinary kinds and uses may be roughly classed into
_fibrous_ and _fleshy_.
71. =Fibrous Roots=, such as those of Indian Corn (Fig. 70), of most
annuals, and of many perennials, serve only for absorption: these are
slender or thread-like. Fine roots of this kind, and the fine branches
which most roots send out are called ROOTLETS.
72. The whole surface of a root absorbs moisture from the soil while
fresh and new; and the newer roots and rootlets are, the more freely do
they imbibe. Accordingly, as long as the plant grows above ground, and
expands fresh foliage, from which moisture largely escapes into the air,
so long it continues to extend and multiply its roots in the soil
beneath, renewing and increasing the fresh surface for absorbing
moisture, in proportion to the demand from above. And when growth ceases
above ground, and the leaves die and fall, or no longer act, then the
roots generally stop growing, and their soft and tender tips harden.
From this period, therefore, until growth begins anew the next spring,
is the best time for transplanting; especially for trees and shrubs.
73. The absorbing surface of young roots is much increased by the
formation, near their tips, of ROOT-HAIRS (Fig. 81, 82), which are
delicate tubular outgrowths from the surface, through the delicate walls
of which moisture is promptly imbibed.
[Illustration: Fig. 83-85. Forms of tap-root.]
74. =Fleshy Roots= are those in which the root becomes a storehouse of
nourishment. Typical roots of this kind are those of such biennials as
the turnip and carrot; in which the food created in the first season's
vegetation is accumulated, to be expended the next season in a vigorous
growth and a rapid development of flowers, fruit, and seed. By the time
the seed is matured the exhausted root dies, and with it the whole
plant.
75. Fleshy roots may be single or multiple. The single root of the
commoner biennials is the primary root, or tap-root, which begins to
thicken in the seedling. Names are given to its shapes, such as
_Conical_, when it thickens most at the crown, or where it joins the
stem, and tapers regularly downwards to a point, as in the Parsnip and
Carrot (Fig. 84);
_Turnip-shaped_ or _napiform_, when greatly thickened above, but
abruptly becoming slender below; as the Turnip (Fig. 83); and
_Spindle-shaped_, or _Fusiform_, when thickest in the middle and
tapering to both ends; as the common Radish (Fig. 85).
76. These examples are of primary roots. It will be seen that turnips,
carrots, and the like, are not pure root throughout; for the caulicle,
from the lower end of which the root grew, partakes of the thickening,
perhaps also some joints of stem above: so the bud-bearing and growing
top is stem.
[Illustration: Fig. 86. Sweet-Potato plant forming thickened roots. Some
in the middle are just beginning to thicken; one at the left has grown
more; one at the right is still larger.]
[Illustration: Fig. 87. Fascicled fusiform roots of a Dahlia: _a_, _a_,
buds on base of stem.]
77. A fine example of secondary roots (67), some of which remain fibrous
for absorption, while a few thicken and store up food for the next
season's growth, is furnished by the Sweet Potato (Fig. 86). As stated
above, these are used for propagation by cuttings; for any part will
produce adventitious buds and shoots. The Dahlia produces _fascicled_
(i. e. clustered) fusiform roots of the same kind, at the base of the
stem (Fig. 87): but these, like most roots, do not produce adventitious
buds. The buds by which Dahlias are propagated belong to the surviving
base of the stem above.
78. =Anomalous Roots=, as they may be called, are those which subserve
other uses than absorption, food-storing, and fixing the plant to the
soil.
_Aerial Roots_, i. e. those that strike from stems in the open air, are
common in moist and warm climates, as in the Mangrove which reaches the
coast of Florida, the Banyan, and, less strikingly, in some herbaceous
plants, such as Sugar Cane, and even in Indian Corn. Such roots reach
the ground at length, or tend to do so.
_Aerial Rootlets_ are abundantly produced by many climbing plants, such
as the Ivy, Poison Ivy, Trumpet Creeper, etc., springing from the side
of stems, which they fasten to trunks of trees, walls, or other
supports. These are used by the plant for climbing.
[Illustration: Fig. 88. Epiphytes of Florida and Georgia, viz.,
Epidendrum conopseum, a small Orchid, and Tillandsia usneoides, the
so-called Long Moss or Black Moss, which is no moss, but a flowering
plant, also _T. recurvata_; on a bough of Live Oak.]
79. =Epiphytes, or Air-Plants= (Fig. 88), are called by the former name
because commonly growing upon the trunks or limbs of other plants; by
the latter because, having no connection with the soil, they must derive
their sustenance from the air only. They have aerial roots, which do not
reach the ground, but are used to fix the plant to the surface upon
which the plant grows: they also take a part in absorbing moisture from
the air.
80. =Parasitic Plants=, of which there are various kinds, strike their
roots, or what answer to roots, into the tissue of foster plants, or
form attachments with their surface, so as to prey upon their juices. Of
this sort is the Mistletoe, the seed of which germinates on the bough
where it falls or is left by birds; and the forming root penetrates the
bark and engrafts itself into the wood, to which it becomes united as
firmly as a natural branch to its parent stem; and indeed the parasite
lives just as if it were a branch of the tree it grows and feeds on. A
most common parasitic herb is the Dodder; which abounds in low grounds
in summer, and coils its long and slender, leafless, yellowish
stems--resembling tangled threads of yarn--round and round the stalks of
other plants; wherever they touch piercing the bark with minute and very
short rootlets in the form of suckers, which draw out the nourishing
juices of the plants laid hold of. Other parasitic plants, like the
Beech-drops and Pine-sap, fasten their roots under ground upon the roots
of neighboring plants, and rob them of their juices.
81. Some plants are partly parasitic; while most of their roots act in
the ordinary way, others make suckers at their tips which grow fast to
the roots of other plants and rob them of nourishment. Some of our
species of Gerardia do this (Fig. 89).
[Illustration: Fig. 89. Roots of Yellow Gerardia, some attached to and
feeding on the root of a Blueberry-bush.]
82. There are phanerogamous plants, like Monotropa or Indian Pipe, the
roots of which feed mainly on decaying vegetable matter in the soil.
These are SAPROPHYTES, and they imitate Mushrooms and other Fungi in
their mode of life.
83. =Duration of Roots, etc.= Roots are said to be either _annual_,
_biennial_, or _perennial_. As respects the first and second, these
terms may be applied either to the root or to the plant.
84. =Annuals=, as the name denotes, live for only one year, generally
for only a part of the year. They are of course herbs; they spring from
the seed, blossom, mature their fruit and seed, and then die, root and
all. Annuals of our temperate climates with severe winters start from
the seed in spring, and perish at or before autumn. Where the winter is
a moist and growing season and the summer is dry, _winter annuals_
prevail; their seeds germinate under autumn or winter rains, grow more
or less during winter, blossom, fructify, and perish in the following
spring or summer. Annuals are fibrous-rooted.
85. =Biennials=, of which the Turnip, Beet, and Carrot are familiar
examples, grow the first season without blossoming, usually thicken
their roots, laying up in them a stock of nourishment, are quiescent
during the winter, but shoot vigorously, blossom, and seed the next
spring or summer, mainly at the expense of the food stored up, and then
die completely. Annuals and biennials flower only once; hence they have
been called _Monocarpic_ (that is, once-fruiting) plants.
86. =Perennials= live and blossom year after year. A perennial herb, in
a temperate or cooler climate, usually dies down to the ground at the
end of the season's growth. But subterranean portions of stem, charged
with buds, survive to renew the development. Shrubs and trees are of
course perennial; even the stems and branches above ground live on and
grow year after year.
87. There are all gradations between annuals and biennials, and between
these and perennials, as also between herbs and shrubs; and the
distinction between shrubs and trees is quite arbitrary. There are
perennial herbs and even shrubs of warm climates which are annuals when
raised in a climate which has a winter,--being destroyed by frost. The
Castor-oil plant is an example. There are perennial herbs of which only
small portions survive, as off-shoots, or, in the Potato, as tubers,
etc.