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 Fic. 61.—Rootstock of a fern (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 
