456 



VENTILATING SYSTEM 



connection between neighbouring girders of the mechanical system. 

 This difficulty is overcome by the interpolation of the aforesaid 



"gutters," which are firmly attached 

 at both ends to the rigid epidermis 

 by means of the thick radial walls 

 of the epidermal cells. Free com- 

 munication between the air-pas- 

 sages and the rest of the venti- 

 lating system is ensured by the 

 fact that the " gutters " are tra- 

 versed by a certain number of 

 intercellular clefts (Fig. 174 i). 

 Some of the features in the 

 structure of hypostomatic air-chambers are connected with the regula- 

 tion of transpiration; in order to avoid needless repetition, these points 

 are reserved for discussion at a later stage (cf. below III. A. 4). 



Fig. 174. 



T.S. through a hypostomatic air-passage in the 

 haulm of EriopAorum alpinum. The hypodermal 

 cells enclosing the passage form a rigid gutter, 

 which is interrupted by an intercellular cleft at i. 

 After Westermaier. 



2. The importance of stomata as paths of gaseous interchange. 



Since the days of Dutrochet, it has been generally recognised that 

 the stomata are [the principal] external openings of the system of 

 intercellular spaces. This view has been subjected to a variety of 

 experimental tests, in all of which the same criterion has been em- 

 ployed, namely, the escape from the cut ends of petiole or stem of air 

 forced inwards through the stomata. Experiments of this kind devised 

 by Dutrochet, Unger, Sachs and many others, are described in every 

 text-book of Plant Physiology. 



The interesting and valuable investigations of Brown and Escombe 222 

 have recently thrown fresh light upon the important question, as to how 

 leaves and other aerial organs are able to carry on a sufficiently active 

 gaseous interchange through such minute pores as the stomata. The 

 experiments carried out by Brown and Escombe upon the diffusion of 

 gases through minute holes in a thin septum, led to the discovery of a 

 very remarkable physical law. It appears that, under the conditions 

 described, the rate of diffusion depends upon the linear dimensions of 

 the holes, and not upon their cross-sectional area. In other words, 

 diffusion goes on more rapidly through very small pores than through 

 larger apertures [with the same total cross-sectional area]. Hence, if 

 a septum is perforated by a sufficient number of minute openings, it 

 would seem as if diffusion should take place as rapidly as if the septum 

 were absent; as a matter of fact, experiment shows that this is actually 

 the case. Brown and Escombe's results thus form a remarkable tribute 

 to the efficiency of the stomata as paths of diffusion. 223 



