ROOTS 
99 
cient moisture from this source, without any soil-relation or 
water-relation. Among these plants the orchids are most 
notable, and they may be observed in almost any green- 
house. Clinging to the trunks of trees, usually imitated 
in the greenhouse by nests of sticks, they send out long 
roots which dangle in the moist air (see Figs. 93, 94). 
It is necessary to have some special absorbing and condens- 
ing arrangement, and in the 
orchids this is usually pro- 
vided by the development of 
a sponge-like tissue about the 
root known as the velamen, 
which greedily absorbs the 
moisture of the air. Examine 
also Figs. 92, 95, 96, 97. 
67. Clinging roots. These 
roots are developed to fasten 
the plant body to some sup- 
port, and do no work of ab- 
sorption (see Fig. 98). Very 
common illustrations may be 
obtained from the ivies, the 
trumpet creeper, etc. These 
roots cling to various supports, 
stone walls, tree trunks, etc., 
by sending minute tendril- 
like branches into the crevices. The sea-weeds 
develop grasping structures extensively, a large majority 
of them being anchored to rocks or to some rigid support 
beneath the water, and their bodies floating free. The 
root-like processes by which this anchorage is secured are 
very prominent in many of the common marine sea-weeds 
(see Fig. 162). 
68. Prop roots. Some roots are developed to prop 
stems or wide-spreading branches. In swampy ground, or 
in tropical forests, it is very common to find the base of 
FIG. 94. An orchid, showing aerial 
roots and thick leaves. 
