86 INTRODUCTION 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 aérial part of 
the plant. It is easier to recog- 
nize as roots such structures as 
the aérial roots of corn and of 
poison ivy than it is to recog- 
nize as stems the thickened 
underground portions of iris, 
jack-in-the-pulpit, dragon-root, trillium, 
or potato. Frequently, like aérial 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 
? C a X s 
Fie. 69. A May-apple plant, showing the history of the rootstock 
lis the oldest surviving portion of the rootstock; 2 is a year younger; 3a 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, 
