INTRODUCTORY SURVEY 19 



the various subordinate parts of which each may be composed, 

 such as bud, petiole or stamen, are some of these organs. 



The organ, in turn, is not homogeneous in texture and con- 

 struction but is made up of a group of tissues, each of which per- 

 forms a particular task contributory to the general function of 

 the entire organ. Thus a stem may be composed of bark tissue, 

 cortical tissue, bast tissue, cambial tissue, woody tissue, and pith 

 tissue, each of them playing some role in the economy of the 

 stem as a whole. One type of tissue may, of course, enter into 

 the construction of several organs; for there is woody tissue, 

 for example, in the root, the stem, and the leaf. 



A tissue, in turn, is an aggregation of cells, those ultimate 

 units of structure and function in all organisms. Each plant 

 cell is a minute but distinct bit of living substance, or protoplasm, 

 with nucleus, plastids, sap-cavity, and other structures of its 

 own, and is usually enclosed in a cellulose wall. The individual 

 plant ma}^, indeed, be regarded as a huge colony of minor indi- 

 viduals, the cells, each performing some particular function in 

 the whole and all bound together to their mutual advantage 

 A knowledge of the structure and activities of cells is the founda- 

 tion upon which our understanding of plants and animals must be 

 built. 



The Root and Its Functions. — The root is that organ which 

 anchors the plant in the soil and which absorbs therefrom water 

 and simple inorganic nutrient materials. Aside from these 

 primary functions, it frequently serves as a storehouse for reserve 

 food and often assumes other secondary duties. The root 

 system may consist of a single strong tap-root, with weak lateral 

 branches, or of a much-branched series of smaller fibrous roots. 

 The function of absorption takes place in minute root-hairs, 

 delicate outgrowths from the surface cells just behind the young 

 and growing root-tip. Into these root-hairs, by the process of 

 os77iosis, pass water and dissolved substances (chiefly nutrient 

 mineral salts) from the soil. 



The Leaf and Its Functions. — The typical leaf is a broad and 

 thin structure, green in color and freely exposed to air and light. 

 Its essential portion, the blade, is usually supported by a stalk, 

 the petiole. From the leaf tissues, the water which has entered 

 the root and ascended the stem is evaporated in the process of 

 transpiration. Most of the internal cells of the leaf possess a 

 green pigment, chlorophijll, which can utilize the energy of hglit 



