44 GROWTH AND WORK OF PLANTS 



68. Principal functions of the stem. The stem has 

 several kinds of work to do. The two principal functions of 

 ordinary stems are, first, support for the leaves, so that they can 

 be well exposed to the light and air, and second, conduction of 

 water and food substances from the roots to the leaves, and of 

 food substances from the leaves to the roots and different parts 

 of the stem. Other functions of stems will be seen in studying 

 the specialized kinds of stems. 



69. Stems respond to the influence of light. They are 

 sensitive to light and under its stimulus they turn toward the 

 light. This is well seen in growing plants like beans, peas, sun- 

 flower, etc., especially in young plants when all the leaves are 

 removed and the plant is placed near a well-lighted window, or 

 in a dark box with a small window at one side. 



70. Peculiarities of stems grown in continued darkness. 

 Stems grown in the dark are very different from those grown in 

 the light. They lack the green coloring matter, chlorophyll, which 

 is formed in the light in leaves and many young stems. The 

 leaves on many such stems are very small, and the stems of many 

 plants are long, more slender, more watery and contain less plant 

 substance than the same plants grown in the light. In the light, 

 building material is formed in the green parts, especially in the 

 leaves. Light also exercises an influence on the form and 

 character of stems and leaves, and under its influence the stems 

 are stockier, shorter and firmer. 



71. Types of stems. There are several types of stems as re- 

 gards the form or habit of the stem system. Some of these types 

 are well shown by different trees. First, excurrent type, three 

 forms of this type are as follows: The conical form is very 

 characteristic of many spruces, the larch, and some other conif- 

 erous trees. The main stem or trunk is straight, continuous 

 through to the topmost part of the tree, and is often called the 

 leader, or the trunk is said to be excurrent. The branches are 

 all lateral to this and much smaller. The lower branches are 

 the longer, and successive branches upward are successively 

 shorter, so that the outline of the tree as a whole is that of a 



