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 

 develop grasping structures extensively, a largo 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. L57). 



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 



94. An orchid, showing aerial 

 roots and thick leaves. 



The sea-weeds (algae) 



