438 VENTILATING SYSTEM 



dependent upon the ventilating system, so far as their gaseous inter- 

 change is concerned. Stahl employed the following method in order 

 to investigate this point. The stomatic lower surfaces of the experi- 

 mental leaves (Primus Padus, Ribcs petraeum, Lonicera tatarica, Phila- 

 delphus coronarius, Impatiens spp.) are partially covered with cacao-wax, 

 so as to prevent the access of carbon dioxide to the underlying sections 

 of the ventilating system. Leaves treated in this manner are exposed 

 to sunlight for several hours ; when subsequently tested by Sachs' iodine 

 method (cf. p. 292), the waxed portions appear yellow, and thus prove 

 to be devoid of starch ; the unwaxed areas, on the other hand, assume a 

 deep blue-black colour. Again, if the entire lower surface of a leaf is 

 covered with cacao-wax, and incisions made on the upper side, so as to 

 provide artificial paths of communication between the ventilating 

 system and the outer air, a considerable quantity of starch is formed 

 in the immediate neighbourhood of each incision, after some hours' 

 exposure to sunlight. Where the smaller veins are embedded in con- 

 ducting parenchyma which extends as far as the epidermis on both 

 sides of the leaf, each of these zones of local starch-formation is limited 

 by the nearest veins ; here the veins evidently act as partitions, which 

 prevent the carbon dioxide from spreading for any considerable distance 

 through the mesophyll. 



In many photosynthetic tissues, the intercellular spaces act as 

 barriers which prevent the synthetic products from diffusing in un- 

 desired directions (cf. p. 282); this subsidiary function has a large 

 share in determining the arrangement of the ventilating spaces within 

 the photosynthetic system. The fact that photosynthetic organs so 

 often contain numerous transversely directed intercellular clefts, an 

 arrangement which renders it impossible for the products of synthesis 

 to travel longitudinally from the first, is really nothing more than an 

 expression of the principle of expeditious translocation ; this principle, 

 which, as we have already seen, plays such a prominent part in the 

 construction of photosynthetic tissues, thus extends its influence over 

 the ventilating system as well. 



3. The ventilating system in relation to transpiration. 



The transpiration-current does not represent the sole vehicle for 

 the transportation of the food-materials which green terrestrial plants 

 absorb from the soil, and hence scarcely possesses the fundamental 

 significance attributed to it by some physiologists ; still, where the 

 climatic conditions are such that plants can transpire actively without 

 continually incurring the risk of excessive loss of water, transpiration 

 does provide a convenient means of accelerating the translocation of 

 nutrient salts. We may therefore expect occasionally to meet with 



