PLANTS AND WATER 141 
of light in a remarkably purposive fashion. 
Thus nearly all the cacti, in their adult stages, 
Fig. 18.—Taeniophylkum torricellense, an orchid with green, 
flattened, leaf-like roots. R, roots; V, vascular strand; 
F, flowering spike. (The roots are shown broken off at 
the ends; they are actually much longer than indicated 
in the diagram, which was made from a specimen in the 
Natural History Museum, 8. Kensington.) 
are destitute of green leaves, their stems 
functioning as the photosynthetic organs. 
Some of them, like the species of Phyllo- 
