508 ECOLOGY 



products of decaying vegetation influence the direction as well as the 

 length of roots. It is likely that the inhibitory influence of these factors 

 increases with the depth ; the greater amount of oxygen in the surface 



FIG. 726. A cypress swamp at low water; note the erect growths (knees) from 

 the roots of the bald cypress (Taxodium) and the buttressed bases of the trunks; 

 intermingled with the cypress (the large trees in the foreground with bark shredding 

 vertically) are specimens of tupelo (Nyssa aquatica; small trees in the background); 

 Paragould, Arkansas. Photograph by MEYERS. 



layers may facilitate growth, in part directly, and in part indirectly, 

 through the partial destruction of deleterious organic compounds by 

 oxidation. 



In a stagnant swamp the surface layer is the only place where at the same 

 time water and oxygen are available for root activity, hence it would seem that hori- 

 zontal roots are best fitted to thrive in such habitats. The explanation of such erect 

 roots or root branches as those of the bald cypress and of the mangroves is 

 difficult. The erect growth certainly is a reaction to some condition in the swamp, 

 since knees do not develop when the bald cypress is cultivated in uplands. Oddly 

 enough, knees do not develop in deep water, but only in shallow water or in swamps. 

 If these peculiar structures are regarded as reactions to a slight oxygen content, 

 it is difficult to account for their absence in deep water, unless it be supposed that 

 the life conditions there are too poor to permit of growth. The erect roots of Jus- 

 siaea are spongy structures resembling aerenchyma (p. 553), and they arise from 

 stems instead of from roots; possibly, like aerenchyma, they develop where trans- 

 piration is checked. 



