STEMS 129 



the result of the action of gravity or light. If it is 

 the result of light, it will generally grow erect in 

 darkness. 



177. Stems are found among the higher plants 

 only. The seed plants and ferns only have true 

 stems as determined by the tissues which compose 

 them. The mosses, liverworts, and some of the sea- 

 weeds have a main axis from which branches are sent 

 off, and while it performs the work of a stem it has 

 not the true stem structures. A body of this kind 

 is termed a thallus. The main axis of the mosses, 

 liverworts, and the larger algae is often designated 

 as a stem, though it does not exhibit the distinction 

 of tissues shown by the true stems of the higher 

 plants. 



