ECOLOGY 



On the other hand, the passage of water and salts from the velarrieri 

 through the ' transfusion cells to the cortex is a slow osmotic process 

 quite comparable to absorption by root hairs. The velamen is an 

 organ of water accumulation as well as an organ of absorption, and* 

 it retains water for hours and even for days. Its significance is still 

 more obvious when it is realized that only liquid water can be utilized 



by plants; orchids even decrease in weight 

 in moist chambers unless watered. Hence 

 it is not surprising that epiphytes with ab- 

 sorptive roots are confined to warm and 

 humid climates, where rain or dew is almost 

 continually available. 



u 



FIG. 734. An outline, as 

 seen in cross section, of a dor- 

 si ventral orchid root (Aeranthus 

 fasciola), showing the expanded 

 upper portion (it), which con- 

 tains most of the chlorophyll. 

 After JANCZEWSKI. 



The chlorophyll which is present in air roots 

 probably is of some importance in food manufac- 

 ture. Doubtless the presence of chlorophyll in these 

 organs is related to light, since various soil roots 

 develop chlorophyll when exposed to it. In Tae- 



niophyllum and in similar orchids with greatly reduced stems and leaves, the 

 roots, which are the chief food-making rgans, are flattened rather than round, 

 and lack the usual radial structure. The lighted side has a thick-walled exoder- 

 mis and prominent cortical chlorophyll but little or no velamen, while the 



shaded side has a strong velamen, a thin- 



walled exodermis, and abundant root hairs 

 (fig. 734). Absorptive air roots play only 

 a small part in anchorage, though in 

 some cases, especially in the flattened roots 

 just cited, they adhere closely to the tree 

 branches. 



- 735- A portion of a progeo- 

 tropic rhizophore of Selaginella apus, 

 densely clothed with horizontal root 

 hairs, as a result of growth in a moist 

 chamber; considerably magnified. 



FIG. 736. A portion of the stem of 

 a liana (Philodendron melanochrysuni) 

 with horizontal adventitious roots clasp- 

 ing the trunk of a tree (Canarium), thus 

 serving to anchor the liana to its sup- 

 port. From WENT. 



