THE STEM 



59 



runners ; but these die in the following winter, if not before, and 

 leave the plants as so many separate individuals. 



91. Subterranean stems and branches. — These are very 

 numerous and various. The vegetation that is carried on 



Rhizome of the Iris. 



45. Rhizomes of the Peppermint. 



underground is hardly less varied 

 or important than that above 

 ground. All their forms may be 

 referred to four principal kinds : 

 namely, the Rhizome^ or Rootstock^ 

 the Tuher^ the Corm or solid bulb, 

 and the true Bulh. 



92. The rootstock, or rhizome, in its 



simplest form, is merely a creeping stem 

 or branch growing beneath the surface of the soil, or partly covered 

 by it (Fig. 45). 



93. Rootstocks are commonly thickened by the storing up of con- 

 siderable nourishing matter in their tissue. The common species of 

 Iris (Fig. 46) in the gardens have stout rootstocks, which are only 

 partly covered by the soil, and which bear foli- 

 age leaves instead of mere scales, closely covering 

 the upper part, while the lower produces roots. 



94. A tuber may be understood to be a por- 

 tion of a rootstock thickened, and with buds 

 (eyes) on the sides. Of course, there are all 



gradations between a tuber 



and a rootstock. Helian- 



thus tuherosus, tlie so-called 



Jerusalem Artichoke (Fig. 



48), and the common 



Potato, are typical and fa- 

 miliar ex- 

 amples of 

 the tuber. 



The stalks by which the tubers 

 48. Tubers of Helianthus tuherosus. are attached to the parent stem 



47. Corm or Caudex, 

 of Trillium. 



