CHAPTER IV 

 THE STEM AND THE LEAF 



39. Stem and leaf as coworkers. In a general way it may 

 be said that the stem and the leaf together do the work of 

 making plant food from the raw materials (Sect. 17). In 

 most of our commonest seed plants the stem is mainly impor- 

 tant as the part of the plant body which bears leaves, maintains 

 them in the- most advantageous position to receive sunlight, 

 carries to them water and dissolved salts from the earth, and 

 carries away from the leaves the newly made plant food which 

 is to serve for the immediate needs of the plant body or to be 

 stored for use later on. The stem and the leaf are so intimately 

 associated that it is often convenient to have a single name 

 for the two together. The stem and its leaves collectively 

 are known as the shoot. 



40. Photosynthesis done by stems. In some practically leaf- 

 less plants, such as the cacti, the photosynthetic work of the 

 plant is all done by the stem, which is covered with layers of 

 chlorophyll-containing cells. Stems flattened so as to expose 

 a good deal of surface for photosynthesis are shown in Fig. 25 

 and still more expanded ones in Fig. 26. In the shrubs known 

 as switch plants (Fig. 365), common in some regions where 

 the summers are hot and almost rainless, the leaves (if there 

 are any) are borne for only a few months of the year, usually 

 in the spring. During the rest of the year photosynthesis is 

 slowly carried on by the green layer of the bark, which is 

 abundantly supplied with chlorophyll. Even among the trees 

 and shrubs of temperate North America there are many 

 species, such as the wahoo, box elder, sassafras, and some 

 roses, which have much green bark on the younger twigs and 

 probably accomplish a good deal of photosynthesis through 



39 



