AERIAL ROOTS 



75 



horizontally they rest on the side walls. Similar readily movable 

 bodies (not always consisting of starch, however) have been 

 found in herbaceous stems which are sensitive to gravity. Since 

 detailed experiments have established that the stimiUus of 

 gravity is most readily perceived in the root-tip,^ some Botanists 



Fig. 33. — Transverse section through part of a root of an epiphytic Orchid 

 (Dendrobiuin), showing the velamen {!'.). Co., cortex ; En., endo- 

 dermis; Ex., exodermis ; P. and Pa., passage cells ; Xy., xylem. 



regard the impact of these faUing starch-grains as the mechanism 

 fof perceiving orientation. Xo adequate proof of this hypothesis 

 has, however, yet been given, the mechanism of gra^'it3'-percep- 

 tion being still unsolved. 



1 Cf. F. &. S, p. 209. 



