DIV. i MORPHOLOGY 175 



volume, exposes the minimum surface and is thus advantageous in 

 diminishing transpiration. NOLL has estimated that the loss of water 

 from a spherical Cactus is 600 times less than from an equally heavy 

 plant of Aristolochia sipho. 



Special interest attaches to some xerophytes in which the stems 

 as well as the leaves are reduced. Thus in the epiphytic orchid 

 Taeniophyllum (Fig. 201) the flattened green roots represent the 

 vegetative organs and carry on the functions of the leaves. 



(e) Adaptations to periodically moist Climates. Tropophytes ( 84 ). 

 In some moist and warm regions of the tropics the climate remains 

 almost equally favourable to the growth of plants throughout the 

 year. Wherever, however, there is a marked periodicity in the 

 climate, with an alternation between a period favourable to the growth 

 of plants and a more or less injurious season, a corresponding 



FIG. 201. Taeniophyllum Zollingeri. A xerophytic orchid without leaf or stem but with green 

 flattened roots. (Xat. size. From SCHIMPER'S Plant-Geography, after WIESNER.) 



PERIODICITY is found in the vital processes of the plants. The resting 

 period may be brought about either by dry ness or by the cold of a 

 winter season. Many of the plants living under such a climate show 

 differences in structure as compared with those of uniformly moist 

 tropical regions. Only those forms will succeed that can endure the 

 unfavourable period in one way or another. The main danger when 

 a cold winter alternates with a summer period lies in death from 

 lack of water during the physiologically dry cold period. This 

 danger does not threaten extreme xerophytes since they are suited 

 to dry habitats in the favourable period, but does affect plants the 

 structure of which is not xerophytic. Since the leaves as the organs 

 of transpiration are especially concerned, the shedding of the leaves 

 before the unfavourable period in the case of deciduous trees or the 

 dying down of the leafy shoots in many herbaceous plants is readily 

 understood. Further, the embryonic tissue, from which the lost parts 

 will be replaced at the commencement of the favourable season, may 

 require to be specially protected from the risk of desiccation. 



