Chapter IX).

=792.= (8) =The maintenance of the required balance between the environment and the increasing or changing requirements of the plant.=—In this matter the entire plant participates. Mention is made here only of the general relation which the root sustains to its own environment and the increased burden placed upon it by the shoot. The increase in the root system keeps pace with the increasing size of the stem system. The roots become stronger, their ramifications wider, and the number of absorbing rootlets more numerous. The observation is sometimes offered that the correlation between the root system of a plant, and the form of the stem system and position of the leaves, is of such a nature that plants with a tap-root system have their leaves so arranged as to shed the water to the center of the system, while plants with a fibrous-root system have their leaves so arranged as to shed the water outward. In support of this attention is called to the radiate type of the leaf system of the dandelion, beet, etc. In the second place the imbricate type as manifested in broad-leaved trees, and in the overlapping branch systems of many pines, etc. One should note, however, that in the former class the leaves are often arranged to shed as much water outward as inward. As to the latter class, there is need of experiment to determine whether these empirical observations are correct, for the following reasons: 1st, Root and leaf distribution are governed by other and more important laws, the root being influenced by the location of food in the soil which usually forms a very thin stratum while the shoot and leaf is mainly influenced by light, and root distribution is much wider in a lateral direction than that of the branches. 2d, In light rains the leaf surface holds back practically all the rain which is then evaporated into the air and lost to the root systems. 3d, In heavy and long-continued rains the water breaks through the leaf system to such an extent that roots under the tree would be as well supplied as those outside, and the ground outside being saturated anyway, the roots do not need the small additional water which may have been shed outward. 4th, It is the habit of plants where left undisturbed (except in rare cases), to grow in more or less dense formations or societies. Here there is no opportunity for any appreciable centrifugal distribution of rainfall and yet the root distribution is practically the same, except that the root systems of adjacent plants are interlaced.

II. Kinds of Roots.

=793. The root system.=—From the foregoing, it will be understood that the roots of a plant taken together form the _root system_ of that plant. In soil-roots in general we usually recognize two kinds of root systems.

=794. The fibrous-root system.=—Roots which are composed of numerous slender branching roots resembling “fibers,” are termed _fibrous_, or the plant is said to have a _fibrous-root system_. The bean, corn, most grasses, and many other plants have fibrous-root systems.

=795. The tap-root system.=—Plants with a recognizable central shaft-like root, more or less thickened and considerably stouter than the lateral roots, are said to have _tap_ roots, or they have a _tap-root system_. The dandelion, beet, carrot (see crown tuber) are examples. The hickory, walnut, and some other trees have very prominent tap-roots when young. The tap-root is maintained in old age, but the lateral roots often become finally as large as the tap-root. Besides tap-roots and fibrous-roots, which include the larger number, several other kinds of roots are to be enumerated.

=796. Aerial roots.=—Aerial roots are most abundantly developed in certain tropical plants, especially in the orchids and aroids. Many examples of these plants are grown in conservatories. The amount of moisture is so great in these tropical regions that the roots are abundantly supplied without the soil relation. Certain of the roots hang free in the air and are provided with a special sheath of spongy tissue called the _velamen_, through which moisture is absorbed from the air. Other roots attach themselves to the trunk or branches of the tree on which the orchid is growing, and furnish the support to the _epiphyte_, as such plants are often called. Among the tangle of these clinging roots falling leaves are caught. Here they decay and nourishing roots grow from the clinging roots into this mass of decaying leaves and supply some of the plant food. Aerial roots sometimes possess chlorophyll.

There are a number of plants, however, in temperate regions which have aerial roots. These are chiefly used to give the stem support as it climbs on trees or on walls. They are sometimes called clinging roots. A common example is the climbing poison-ivy (Rhus radicans), the trumpet creeper, etc. Such aerial roots are called _adventitious_ roots.

=797. Bracing roots, or prop roots.=—These are developed in a great variety of plants and serve to brace or prop the plant where the fibrous-root system is insufficient to support the heavy shoot system, or the shoot system branches so widely props are needed to hold up the branches. In the common Indian corn several whorls of bracing roots arise from the nodes near the ground and extend outward and downward to the ground, though the upper whorls do not always succeed in reaching the ground. The screw-pine so common in greenhouses affords an excellent example of prop roots. The roots are quite large, and long before the root reaches the soil the large root cap is evident. The banyan tree of India is a classic example of prop roots for supporting the wide-reaching branches. The mangrove in our own subtropical forests of Florida is a nearer example.

=798. Buttresses= are formed at the junction of the root and trunk, and therefore are part root and part stem. Splendid examples of buttresses are formed on the silk-cotton tree. They are sometimes formed on the elm and other trees in low swampy ground.

=799. Fleshy roots, or root tubers.=—These are enlargements of the root in the form of tubers, as in the sweet potato, the dahlia, etc. They are storage reservoirs for food. Portions of the roots become thick and fleshy and contain large quantities of sugar, as in the sweet potato, or of _inulin_ (a carbohydrate) in the root tubers of the dahlia and other composites.

=800. Water-roots and roots of water plants.=—These are roots which are developed in the water, or in the soil. Water-roots are sometimes formed on land plants where the root comes in contact with a body of water, or a stream. Water-roots usually possess no root hairs, or but a few, as can be seen by comparing water-roots with soil-roots, or by comparing roots of plants grown in water cultures. The greater body of water in contact with the root and the more delicate epidermis of the root render less necessary the root hairs. The duck-meats (Lemna) are good examples of plants having only water-roots. Other aquatic plants like the potamogetons, etc., have true roots which grow into the soil and serve to anchor the plant, but they are not developed as special organs of absorption, since the stem and leaves largely perform this function.

=801. Holdfasts.=—These are organs for anchorage which are not true roots. These are especially well developed in some of the algæ (Fucus, Laminaria, etc.). They are usually called _holdfasts_. The holdfasts of the larger algæ are mainly for anchoring the plant. They do not function as absorbing organs, and the structure is different from that of true roots.

=802. Haustoria or suckers= is a name applied to another kind of holdfast employed by parasitic plants. In the dodder the haustorium penetrates the tissue of the _host_ (the plant on which the parasite grows), and besides furnishing a means of attachment, it serves as an absorbing organ by means of which the parasite absorbs food from its host. The parasitic fungi like the powdery mildews which grow on the surface of their hosts have simple haustoria which serve both as organs of attachment and absorption, while in the rusts which grow in the interior of their hosts the haustoria are merely absorbing organs.

=803. Rootlets, or rhizoids.=—Many of the algæ, liverworts and mosses have slender, hair-like organs of attachment and absorption. These plants do not have true roots. Because of the slender form and small size of these organs, they are called _rhizoids_, or _rootlets_. In form many of them resemble the root hairs of higher plants.