86 



1NTKODUCTION TO BOTANY 



Desert plants, or those which for any reason may often be 

 exposed to intensely hot, dry air, are safer without large air cavi- 

 ties anywhere in the interior of the plant, but stored water is of 

 the greatest use to such plants, and it may be found in the 

 roots (fig. 20), in the stem (fig. 66), or in the leaves (fig. 65). 

 84. Characteristics of underground stems. The popular 

 notion of a stem includes the idea that it is an aerial part of 



the plant. It is easier to recog- 

 nize as roots such structures as 

 I v \M&. ^ e a ^ r * a l ro ts of corn and of 

 \IWlj|k poison ivy than it is to recog- 

 H ^ nize as stems the thickened 

 underground portions of iris, 

 jack-in-the-pulpit, dragon-root, trillium, 

 or potato. Frequently, like aerial stems, 

 underground stems are divided into 

 nodes and internodes ; many of them 

 bear scales which represent leaves, and 

 in the axils of these scales they produce 



FIG. 69. A May-apple plant, showing the history of the rootstock 



1 is the oldest surviving portion of the rootstock ; 2 is a year younger ; 3 a year 

 younger than 2, and so on. At each figure the cluster of roots marks the position 

 of the base of the upright stem for that year, as is shown at 6. b, bud for the new 

 year's growth ; br, bract at the base of the present stem. One sixth natural size 



buds. Such buds are well shown in the underground stems 

 of some grasses. Dicotyledonous underground stems usually 

 have distinct bark, wood, and pith ; most dicotyledonous roots 

 do not have pith, though some do. 



Some of the principal forms of underground stems have for 

 convenience been given special names. The elongated forms, 



