212 



BOTANY 



PART I 



Gases diffuse through cell-walls more easily the richer in water 

 these are. The ordinary cell-wall, Avhen in a dry condition, hardly 

 allows gases to dilTuse through it ; in nature, however, the cell-Avall 

 is always more or less saturated with water. The cuticle, on the other 

 hand, has very little power of imbibing water, and places considerable 

 difficulty in the way of any diosmotic passage of gases ; it is not, how- 

 ever, completely impermeable. 



Practically the gaseous diffusion takes place rather through the 

 substances with which the cell-wall is impregnated than through the 



substance of the wall itself. 

 Since carbon dioxide is much 

 more readily soluble in water 

 than is oxygen, it will be 

 evident that it will pass more 

 rapidl}^ thi'ough a cell-wall 

 saturated with water than 

 oxygen will. In all proba- 

 bility this holds for the cuticle 

 as well. Since, however, the 

 partial pressure of the oxygen 

 in the air • is relatively con- 

 siderable, while that of carbon 

 dioxide is very slight, oxygen 

 can pass in sufficient quantity 

 through the cuticle but not 

 carbon dioxide ; on this account 

 we find that all organs which 

 only require to absorb oxygen 

 are unprovided with stomata, 

 while organs which absorb 

 carbon dioxide always have 

 stomata. 



lu the soil as well as in the air, 

 jilants, as a rule, find so much oxygen 

 that this gas is able to pass through 

 the cuticle. Organs which live in 

 swampy soil which is poor in oxygen 

 form an exception to this. In marsh- 

 plants, which stand partly in the air, 

 the large intercellular spaces form connecting canals through which the atmospheric 

 oxygen without being completely used up can reach the organs growing deep in 

 the swampy soil and cut oil' from otlier .supi)lies of oxygen. In some cases the need 

 of a supply of oxygen to such roots is met by specialised roots (pneumatophoiies) 

 Avhich project vertically from the muddy soil (Fig. 193), and absorb oxygen from 

 the air either through their cuticle or by special openings. 



The efficiency of tlu; stomata in gaseous exchange varies with the width to 

 which the por(!S are open. The closure of the jiores of the stomata, which may be 



Fig. VM. — Respiratory root of Avicennia, one of the 

 Mangroves. (5 nat. size ; from Schimper's Plant- 

 Geography.) 



