28 



TROPICAL AGRICULTURE 



CHAP 



food, 



and then 

 perish, 



when fresh 

 ones take 

 their place. 



Root hairs, off minute, hair-like prolongations, called appropriately root- 

 hairs. The extremity of a root is covered by a cap of 

 stronger tissue so as to protect the growing point, and to 

 enable it to make its way into the soil. The root-hairs apply 

 themselves to the particles of the soil, and by means of the 

 ? acid fluid permeating their very thin walls, they dissolve 



ikeuppiLt out the active constituents, and then take up and pass them 

 on to the sap, which is used for the growth and nourishment 

 of the plant. The root-hairs perish as soon as they have 

 extracted the soluble portions of the soil they are applied to, 

 but fresh ones are put out from more advanced parts of the 

 roots, so as to seek for the nutritive substances from other 

 particles of the soil, and thus this process goes on continu- 

 ally, and the earth is searched in all directions for the food 

 necessary for the well-being of the plant. 



THE STEM is sometimes called the ascending axis of the 

 plant, because, when the seed begins to develop, the 

 minute stem always strikes up towards the air and light, 

 whilst the roots strike downwards into the soil, avoiding 

 light and air. In certain instances, however, the stem runs 

 along the ground as in some grasses and other plants, or it 

 may even run underneath the surface, thereby resembling 

 the root. 



THE LEAVES are flattened expansions of the stem, in 

 which some of the most important functions of the plant 

 take place. They are composed of a thin layer of vegetable 

 tissue called parenchyma, held together by ribs or frame- 

 work of wood-like substance wrongly called veins, and 

 covered on both surfaces by a thin skin of much finer 

 material known as the epidermis. In the epidermis there 

 are pores or openings called stomata., through which car- 

 bonic acid, oxygen and watery vapour are taken in or given 

 out by the leaf. The functions of leaves have been de- 

 scribed as a combination of those of the lungs and stomach 

 of animals, for the leaves not only take in gases and after- 



importance 



of leaves. 



Their 

 functions. 



