512 



ECOLOGY 



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

 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. 



FIG. 734. An outline, as 

 seen in cross section, of a dor- 

 siventral orchid root (Aeranthus 

 fasciola), showing the expanded 

 upper portion (u), 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 Organs, 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 

 thamber; considerably magnified. 



FIG. 736. A portion of the stem of 

 a liana (Philodendron melanochrysum) 

 with horizontal adventitious roots clasp- 

 ing the trunk of a tree (Canarium), thus 

 serving to anchor the liana to its sup- 

 port. From WENT. 



