ROOTS AND RHIZOIDS 



513 



Variations. — The high specialization and the obvious role of absorptive air 

 roots give the question of their origin much interest, but the lack of experimental 

 data makes the solution of the problem very difficult. The most characteristic 

 feature of these roots, the vela- 

 men, is possessed by nearly all 

 aerial roots and by almost no 

 soil roots, except in a few forms, 

 such as Blelia and Spiranihes 

 cernua; thus the medium seems 

 to have a rather definite relation 

 to the formation of the velamen. 

 The most obvious reaction of 

 air roots to changed conditions 

 is in hair production. Most 

 air roots are hairless ; some (as 

 in species of Anthurium) com- 

 monly possess hairs, and root 

 hairs develop in a number of 

 species when the roots are ex- 

 posed to unusually moist air 

 (fig. 735). A curious exception 

 to the usual kind of root hair 

 is found in some air roots, where 

 the hairs are stiff and rigid 

 structures of long duration. 



Anchoring air roots. — 



Some climbing plants, as 

 poison ivy and English ivy, 

 are anchored to the sup- 

 porting trees by adventi- 

 tious roots, which clasp the 

 trunk or penetrate into 

 bark furrows. Such roots 

 grow horizontally rather 

 than downward (fig. 736), 

 notably in certain tropical 

 climbers, whose roots are as 

 sensitive to contact stimuli 



Figs. 737, 738. — ^Root cross sections of Philoden- 

 dron lacerum, a tropical liana: 737, a nutritive root 

 with many large conductive vessels (■y), outside of 

 which is a sheath of thick- walled sclerotic cells {s), 

 cortical parenchyma (c), and epidermis (e) ; 738, an 

 anchoring root with much smaller vessels (i/) and 

 with a much thicker sclerotic sheath {s) ; the nutritive 

 root is much the larger, the two figures being equally 

 magnified. — From Went. 



as are many tendrils. If 



these roots are at all geotropic, the bark moisture and other influences are 

 sufficient to overcome gravity and to induce lateral growth. CHmbing 

 roots commonly are supposed to be anchoring roots only and riot absorp- 



