162 STEMS 



of the intense struggle for light in tropical forests. Not 

 only are myriads of lianas to be seen there, but orchids and 

 other epiphytes grow upon the branches of the trees. Some- 

 times the trees support dozens of different kinds of stems 

 and leaves besides their own ; advantage seems to be taken 

 of every inch of space available for leaf work. (Epiphytes 

 are plants which grow, root and all, upon other plants; 

 epi means upon; phyte means plant.) Similarly in our 

 own forests there is competition for light, and the vernal 

 habit appears to be a result of this competition. In our 

 own forests the struggle for light is not so continuous as 

 in the tropics ; there the struggle goes on month after 

 month throughout the year, and at no season do low 

 plants have much chance. With us, however, there is a 

 growing season of about thirty days before the trees are in 

 full foliage, and at this season the low plants of the forest 

 have their best chance ; they take advantage of this brief 

 period when sunshine strikes down to the forest floor. 



Most of the plants usually studied by high school classes 

 on field trips in the spring are plants which arise from 

 bulbs or rhizomes or corms and have the vernal habit. 



So. The Structure and Growth of Stems. — This divi- 

 sion of the study of stems is a little more difficult to under- 

 stand than what has preceded. We have been discussing 

 stems as to their outside, and with the outside of stems 

 you were already somewhat familiar. We now must dis- 

 cuss stems as to their inside, and with the inside of stems 

 you are not familiar. We have been discussing things which 

 you can see easily with the naked eye. We now must dis- 

 cuss things which it takes a microscope to see. No matter 

 how clear the description, facts which it takes a microscope 



