122 METABOLISM 



different conclusion ; an elliptical opening behaves exactly in the same way 

 with regard to diffusion as a circular opening of the same area, each non-circular 

 opening must be first of all reduced to a surface area of circular outline and the 

 diameter of the latter taken as the figure on which diffusion depends. Thus 

 Brown and Escombe estimated the effective size of the opening between the 

 guard-cells of the stoma of Helianthus at 0-0000908 sq. mm., corresponding to 

 a circular area of 0-0107 "i^i- ^^ diameter. The distance of the stomata apart is 

 equal to about eight times their diameter ; they may thus interfere with each 

 other, but only slightly. If we assume that the absorption of carbon-dioxide 

 by the mesophyll is complete, we may conclude from the number of the stomata 

 that 2-095 ccm. could be absorbed per sq. cm. every hour. As a matter of fact, 

 the leaf absorbs only 0-134 ccm. per sq. cm., in order to construct a maximum 

 of 1-8 g. of carbohydrate per sq. m., that is to say, only about 6 per cent, of 

 the amount theoretically possible. From this it follows that the carbon- 

 dioxide can only be absorbed slowly — for it must first pass through the cell- walls 

 — so that the partial pressure a short distance below the stoma is far from 

 being reduced to zero, while in physical experiments this condition is rapidly 

 attained. ' The structure of a typical herbaceous leaf ', says Brown, ' is an 

 admirable example of adaptation to natural laws, which in this particular in- 

 stance have only recently become known to us. It illustrates in a striking 

 manner all the physical properties of the multiperforate diaphragm, which, 

 with its minute apertures, set at from six to eight diameters apart, and repre- 

 senting only I per cent, to 3 per cent, of free area, yet allows a perfectly free 

 interchange of gases on its two sides, whilst at the same time affording every 

 protection to the delicate structures underlying it.' 



The stomata, however, are by no means always wide open, the size of the 

 opening, on the contrary, varies with external conditions as we pointed out 

 when discussing transpiration Here, as on the previous occasion, we will 

 content ourselves with referring to the two most important factors in the case, 

 viz. light and atmospheric moisture. Since the maximum opening of stomata 

 occurs in bright light we must recognize that as an adaptation of funda- 

 mental importance in the process of carbon assimilation. Increase of assimila- 

 tion accompanies increase in light intensity (p. 125), provided that carbon-dioxide 

 be present in sufficient quantity. An increase of atmospheric moisture brings 

 about opening, a decrease a closing of the stomata, as has been already shown. 

 Often long before a visible wilting of the leaf has taken place a complete closure 

 of the stomatal slit is effected, as a result of the arrangement of the guard-cells 

 already described. This closure of the stoma is essential to the preservation of 

 life, otherwise withering must at once result ; it is a safeguard against excessive 

 transpiration, although at the same time a disadvantage so far as assimilation is 

 concerned. It has been demonstrated by many experiments to how great an extent 

 assimilation depends on the amount of water in the leaves ; thus Kreussler 

 (1885) observed that isolated branches, when brightly illuminated, showed a 

 rapid decrease in the power of assimilation, while, according to Nagamatz (1887), 

 withered leaves lose it altogether. In this case the closure of the slit is alone 

 responsible for the deficiency in carbon-dioxide, and it must not be thought that 

 assimilation depends on the existence of a certain degree of osmotic pressure in 

 the interior of the cells. Assimilation is not lowered owing to loss of water in the 

 case of plants such as mosses and lichens, which absorb carbon-dioxide by means 

 of their cell-walls, so much as it is in the case of foliage leaves (Bastit, 1891, 522 ; 

 JUMELLE, 1892, 166), and in Algae assimilation has been observed to continue 

 even after the beginning of plasmolysis (Klebs, 1888). It must not, on the other 

 hand, be affirmed that the amount of water present in the cell as such is a matter 

 of no moment in the process of assimilation, for Jumelle's researches show that 

 even in lichens assimilation is undoubtedly dependent on the amount of water 

 present. 



