STEMS 



63 



leaf-bearing structure, it continues to bear leaves when 

 under ground; but often these leaves are much modified, 

 either reduced in size so as to be mere rudiments, or used 

 for some other purpose. The fact that a subterranean 

 structure bears leaves of some kind indicates that it is a 

 stem and not a root. Since both the stem and its leaves 

 must be considered in connection with the underground 

 habit, the shoot (21) will be considered rather than the 

 stem alone. In general the subterranean shoot is con- 

 spicuously a region of food storage. The three following 

 types are the most common. 



(4) Rhizomes. This is probably the most common 

 form of subterranean stem. It is usually horizontal, more 

 or less elongated, and much 

 thickened for food storage, and 

 is often called the rootstock 

 (Fig. 61). It advances through 

 the soil year after year, often 

 branching, sending out roots be- 

 neath and leaf-bearing branches 

 into the air. As it continues to 

 grow at the apex, it gradually 

 dies behind, 

 thus isolating 

 branches in 

 the case of 

 branching rhi- 

 zomes. It is 

 a very efficient ? 



method for the ^ IG ' 61- R otstock of a f ern (common brake), 



bearing young leaves. 



spreading of 



plants and is extensively used by grasses in covering areas 

 and forming turf. The persistent continuance of some 

 weeds, especially certain grasses and sedges that infest 

 lawns and meadows, is due to this habit (Fig. 62). It is 



