32 



from them. Wieler's 1 contention that the passages are 

 plugged and cannot serve as gas channels applies, at 

 best, only to the case examined by him and does not 

 weaken the conclusion that, in general, the pneumoto- 

 phores are really organs of gas exchange. In the case of 

 the mangrove and similar trees it is not improbable that 

 gas diffusion is assisted by temperature changes caus- 

 ing convectional gas circulation or by expansions and 

 contractions due to rise and fall of tides 2 (with 

 varying external pressure on the roots) or to wind-strains 

 communicated mechanically from the trunk. 



But, while the existence of pneumatophores may 

 be regarded as evidence of the need of aerobic root 

 respiration on the part of the species which possess them, 

 this evidence applies, obviously, only to those species. 

 As a matter of fact pneumatophores of any type are rare. 

 By far the larger number of swamp plants have neither 

 pneumatophores or any other type of determinable organ 



1. Jahrb. wiss. Bot. 32: 503 (1898). 



2. Westermaier,- Zur kenntnis der Pneumatophoren, 

 P» (1900). On internal spring-like hairs which 

 cause expansion after compression see Gurtler,- Interzell. 

 Haarbildungen, 1905. 



