Ecological Groups of Stems 159 



Underground stems. Many plants belonging to each of the 

 three ecological groups, xerophytes, mesophytes, and hydro- 

 phytes, possess imderground stems. Underground stems are 

 particularly useful as storage places for accumulated food and 

 water, and as organs for propagating the plant (page 217). 



The commonest type of underground stem is the rootstock. 

 Rootstocks are horizontally growing stems, from which the 

 aerial stems arise. They may be slender, or thick and fleshy. 

 Usually they have small scale leaves and buds at the nodes, 

 and roots that arise from the nodes or from the entire imder 

 surface. The presence of nodes is the external feature of 

 underground stems that distinguishes them from roots. 



In many of the grasses and grasslike plants rootstocks de- 

 velop rapidly in aU directions, sending up erect branches at 

 short intervals. The rootstocks and their accompanying 

 roots soon become mixed with those of adjoining plants, 

 finally forming a closely interwoven mat which is the " turf " 

 of lawns and meadows. Turf-forming grasses are often of 

 great value for holding in place the soil of embankments, 

 dikes, and levees. In these plants the rootstocks are mainly 

 useful in spreading or extending the plant. Bermuda grass 

 and Johnson grass are sometimes troublesome weeds because 

 of their extensive rootstock system. The sand-reed grass 

 has been planted extensively in Europe and in America to 

 hold drifting sand in place, and to prevent the sand from in- 

 vading towns and cultivated fields. 



In plants like the May apple, Solomon's seal, and yeUow 

 water Kly, the rootstock not only causes the plant to spread, 

 but it also accumulates a part of the food manufactured each 

 season and thus serves as a storage organ. It is this store of 

 food and the readiness with which the rootstock sends up 



