74 THE FACTORS [Part I 



they are branched like coral and surrounded by a thick snow-white coating 

 of aerenchyma. 



Lateral roots that serve as pneumatophores of various patterns occur 

 in many other plants. They are not always submerged, but in the 

 majority of cases, at least periodically, project into the air, and accordingly 

 then possess characters other than those of submerged types. These 

 projecting respiratory roots are firm in structure, their aerating tissue is not 

 aerenchyma, but air-containing cork or cortex, and their upright position 

 is not passive, but active and due to negative geotropism. Such pneu- 

 matophores frequently attain considerable dimensions, like those of 

 Eugeissona tristis, a palm growing on wet soil, in the case of which they 

 attain a height of i \ m. and a diameter of 3-5 cm. ; or again the peculiar 

 ' knees ' of the swamp cypress, Taxodium distichum (Fig. 48), which 

 resembling sugar-loaves in shape and size, project from the frequently 

 inundated southern swamps of North America ; or still again the variously 

 modified root-structures of shrubs and trees of the mangrove- swamps. 

 These will be described in a subsequent chapter when the tropical 

 formations are dealt with. 



The oecological importance of aerating tissues and pneumatophores 

 has up to the present time been studied chiefly on morphological grounds, 

 and would therefore have remained hypothetical, had not G. Karsten and 

 Greshoff demonstrated it in one case, namely, in the pneumatophores of 

 Bruguiera eriopetala, at the Buitenzorg botanic garden. The pneumato- 

 phore on which they experimented exhibited ' an extremely great working- 

 power,' namely, a very strong excretion of carbon dioxide (once, over 45 c.c. 

 in an hour), which, as was shown by a comparison with the respiration of 

 the whole root-system of a young plant, would be quite inexplicable ' if we 

 wished to refer the result obtained only to the part of the root that was 

 exposed to daylight.' Only the assumption that the root, of which the 

 action was investigated, served as an excretory organ for a larger part of 

 the root-system, can explain the high figures obtained. 



3. THE WIND. 



The vegetation of windy regions exhibits many peculiarities, which may 

 be explained partly as due to direct action of the wind and partly as 

 adaptations to withstand it. These effects of air-currents are apparent 

 both in the vegetative and in the reproductive organs of plants. 



1. Wind and Tree-growth. 



Areas with an atmosphere almost constantly in active movement, such 

 as fiat coasts and islands which experience the first impact of the sea- 

 breezes, or elevated unsheltered mountain ridges, are usually characterized 



