132 



CELLS AND TISSUES 



which spread into the soil where they take up water by means 

 of osmosis, are the chief absorptive structures. (Fig. 119.) There 

 are some plants, however, which live on other plants, in which 

 case the root tissues absorb directly from the tissues with which 

 they are in contact. In some cases the leaves absorb, as in 

 the Sundew (Drosera), Venus's Flytrap (Dioncea muscipula), 

 and Pitcher Plants (Sarracenia) , where the leaves are especially 

 constructed for catching and absorbing insects. . 



Food-making Tissues. The principal food-making organs are 

 the leaves where the cells are provided with chloroplasts and so 

 arranged that they can obtain the raw materials from which foods 



r-e 



FIG. 120. Cross section of the leaf. /, food-making tissue; e, epidermis; 

 y, cross section of a vein* 



are made. (Fig. 120.) However, food-manufacture is not lim- 

 ited to leaves, for all green stems have just under their epidermis 

 a band of green cells, known as the cortex, in which food is manu- 

 factured as long as light and air are not excluded. (Fig. 117.) 



Storage Tissues. Any living cell usually contains some 

 stored food, but there are cells which have food storage as their 

 chief function. This is true in the endosperm and fleshy cotyle- 

 dons of seeds, and in Irish Potatoes, Sweet Potatoes, and in other 

 tubers and roots where the cells enlarge and become packed with 

 food. In the pith some water is usually stored and often much 

 food, as is well known in the case of Sugar Cane and Sorghum in 

 which the pith contains much sugar. Throughout the wood of 

 trees there are thin- walled living cells, forming the medullary rays, 

 which function as a storage tissue. In Maple trees the sugar 

 occurring in the spring sap comes from the starch which was 



