STEMS 



231 



The most impressive stem in relation to leaf display, both 

 in character and amount, and also in its construction for 

 maintaining a rigidly erect position, is the tree. But trees 

 differ in the completeness of provision for leaf display. In 

 some trees, as the pines and their allies (Fig. 195), the main 

 stem continues as a central shaft to the top, the branches- 

 spreading horizontally from it, resulting in a general conical 

 outline. In other trees, as the elm (Fig. 196) and the oak 

 (Fig. 197), the main stem soon divides into branches, 

 and there is a great spreading top or " crown," which 

 represents the most complete provision 

 for a maximum amount of leaf surface 

 well displayed. 



FIG. 199. A strawberry plant, showing a runner that has de- 

 veloped a new plant, which in turn has sent out another 

 runner. After SEUBERT. 



There are certain conditions, however, in which the free 

 erect position of aerial stems is not maintained. In some 

 plants the stem or certain of its branches lie prostrate on the 

 ground or nearly so, sometimes spreading in all directions 

 and becoming interwoven into a mat or carpet (Fig. 198). 

 Such plants grow in general on sterile and exposed soil, and 

 there may be an important relation between this fact and 

 their habit. A prostrate stem is in a distinctly disadvantage- 

 ous position foFleaf display, for instead of being free to 

 spread its leaves out in all directions, the free space for leaves 

 is diminished at least one-half. Although freedom for leaf- 

 display is restricted, prostrate stems are in a very favorable 



