NATURE OF PLANTS 113 



storage of food, as in the bulb type of stems of lilies, onions, etc. 

 (Fig. 73, B). 



It is noteworthy that these special types of stems, the runner, 

 rhizome, bulb, etc., are the most common. They represent the 

 more recently evolved forms of stems and because of their ad- 

 vantages they have become the dominant forms. The geological 



FIG. 73. Shortened types of stems: A, corm of jack-in-the-pulpit. At 

 left surface view showing lateral buds, roots and sheathing leaf arising from 

 top of shortened stem. At right .sectional view with folded leaf, /, in bud at 

 apex of stem. B, bulb type of shortened stems. At left bulb of onion showing 

 the ensheathing leaves which 3re swollen at their bases with food, thus forming 

 the bulb. At right, section of a bulb of hyacinth showing the fleshy leaves 

 attached to the very short stem and in the center of the bulb a flower cluster. 



ancestors of our seed plants were woody and tree like. The 

 uniform climatic conditions of these past periods favored the 

 tree type of stem with its admirable devices for leaf display. 

 With the appearance of climatic changes upon the earth there 

 resulted modifications in the stem : the medullary rays tended to 

 8 



