58 BIOLOGY OF PLANTS. 



2. Siih-soil. The sub-soil lies immediately below the soil, and 

 is mostly destitute of vegetable matter. 



The parts of plants which are concerned in nutrition are the 

 root, stem and leaves. 



1. Root. The root is that part of the plant Fig. 11. 



which penetrates the soil. The following are 

 some of the different varieties of roots : tap 

 roots, as in lucern and clover ; spindle roots, 

 as in tlie carrot, parsnip and beet ; branching 

 roots, as in most forest trees ; fibrous roots, 

 as in the grasses and most annual plants ; creep- 

 ing roots, as in the strawberry ; tuberous roots, 

 as the potato, and bulbous roots, as in the 

 fleshy plants, the onion, turnip, (Fig. 11, a,) which are composed of 

 regular concentric layers of vegetable matter. Roots increase 

 in length by the addition of matter to their points. When this 

 matter is first added it is soft, and possesses the properties of a 

 sponge, to absorb the gaseous, or liquid bodies, which are pre- 

 sented to it. On this account the points b, are called sponge- 

 lets or spongioles. It is through these, that most of the nour- 

 ishment derived from the soil, is conveyed into the organs of 

 the plant. The roots are also supposed to excrete matter into 

 the soil, which having passed through all the transformations 

 it is capable of in its descent from the leaves, is now rejected 

 as unfitted to nourish the plant. The root is also supposed to 

 have the power of selecting those substances which the wants 

 of the Yjlant require, as the same species will absorb unequal 

 quantities of different substances wlien presented to them. 

 But the discriminating and excretory power has been doubted, 

 and these functions of the root are not yet fully established. 



2. Stem or Culm. The stem (Fig. 12) is made up of bundles of 

 small tubes, extending from the roots to the leaves, in which sap 

 and air circulate. The bark coutains similar tubes for the de- 

 scent from the leaves of the cambium, or elaborated juices. 

 The stem contains the pith c, which consists of tubes disposed 

 horizontally, and forming by the medullary rays, a communica- 

 tion with the bark ; but so far as experiments have been tried 

 with colored solutions, the pith does not serve the purpose of 

 circulating the sap. The tubes of the wood aid the ascent of 

 the sap, which, in its progress upward, is sid)jected lo certain 

 chemical changes, and it is supposed by some that the various 

 gaseous bodies, which <ire in the wood, and ascend to the leaves, 

 are produced by transformations in the c>ap in its progress up- 



