114 EVOLUTION OF STEM TYPES 



increase in size and so serve as a storehouse, thus meeting the 

 greater need of reserve material; the cambium also tended to 

 form more and more parenchymatous tissue so that the stem 

 became less woody and more of a storage organ. So you can 

 think of the stem becoming reduced in size and finally prostrate 

 through lack of supporting material. The contact of the stem 

 •with the soil exposed it to a variety of new factors which lead to 

 further modifications, some of which have survived as seen in 

 the runner, rhizome, corm as well as in the annual and biennial 

 habit of plants. It is a striking fact that the majority of our 

 primitive plants are woody and tree like (pines, willows and oaks) 

 while the more recently evolved and higher types (daisies, 

 dandelions and golden-rods) are characterized by various forms 

 of these modified stems. 



