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 photosynthests — 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 grew 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-like and food 
is made chiefly by the stem and its many, small, lateral branches. 
Some plants which have scale-like 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, 
