WORK DONE BY STEMS 169 



As a Conductive structure the stem occupies an important posi- 

 tion, for through it the leaves and roots exchange materials. 

 Consequently, the vascular bundles, forming a continuous con- 

 ductive system from roots to leaves, are prominent structures in 

 stems. Through the conductive system the leaves receive water 

 and mineral salts from the soil and the roots receive the food made 

 in the leaves. For this reason any injury to the stem, such as 

 girdling, which severs the conductive system is likely to seriously 

 injure the plant. In fact, girdling is a common method employed 

 in killing trees. 



In the manufacture of plant foods stems may assume consider- 

 able importance, although seldom so much as leaves, which have 

 food-making as their primary function. Being well exposed to 

 light and well provided with chlorophyll, leaves are especially 

 adapted to carry on photosynthesis — the process by which food is 

 manufactured. However, any portion of a plant containing chlo- 

 rophyll to which sunlight and air are accessible can make food, 

 and the stems of practically all plants that make their own food 

 have some portions that are green and, therefore, able in some de- 

 gree to carry on photosynthesis. For example, the young twigs 

 of trees are almost as green as the leaves and no doubt make con- 

 siderable food. As the twigs grow older, the green layer is cov- 

 ered by bark, which excludes the light that is necessary for 

 photosynthesis. In the Box-elder, Sassafras, and some other 

 trees, not only the young twigs but portions of the older branches 

 are green, and probably able to make food. In most of our 

 short-lived plants, such as Corn, Sorghum, Kafir Corn, Tomatoes, 

 Melons, Clover, Alfalfa, Beans, etc., the entire stem is green and 

 able to carry on photosynthesis. In some plants, such as the 

 Cacti, which have no leaves, all of the food must be made by the 

 stem. In the garden Asparagus the leaves are scale-hke and food 

 is made chiefly by the stem and its many, small, lateral branches. 

 Some plants which have scale-hke leaves, have green lateral 

 branches so expanded as to resemble leaves, as the Smilax 

 {Myrsiphyllum) , common in greenhouses, illustrates. Such 

 branches are called Cladophylls. (Fig. 147.) 



As to the storage function of stems, there is much difference 

 among plants, but in nearly all stems there is some accumulation 

 of substances, such as water, sugars, and starch. During the wet 

 season the stems of some Cacti take up large amounts of water, 



