38 GROWTH AND WORK OF PLANTS 



61. 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 tn the leaves, and of 

 food substances from the leaves to the roots and different parts 

 ofjlje-stem. Other functions of stems will be seen in stuHymg 

 the specialized kinds of stems. 



62. 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. 



63. Peculiarities of stems grown in continued darkness. 

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

 the light. They lack chlorophyll, the green coloring matter 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. This fur- 

 nishes the substance for building material especially of the cell 

 wall, the firm and hard parts of the stem. Thus under the 

 influence of light the stems are stockier, shorter and firmer. 



64. Types of stems. There are several types of stems as 

 regards the form or habit of the stem system. Some of these 

 types are well shown by different trees. Observations can be 

 made on these in the fields, parks and woods. The conical type 

 is very characteristic of many spruces, the larch, and some 

 other coniferous trees. The main stem or trunk is straight, con- 

 tinuous 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 



