120 METABOLISM 



It will be observed that the rate of assimilation increases markedly from the 

 very commencement, and that when the percentage of carbon-dioxide is thirty- 

 five times the normal (i.e. about i per cent.) an optimum condition is reached. 

 GoDLEWSKi's (1873) figures, frequently at all events, show more strikingly than 

 those of Kreussler (whose last figure is doubtful) an obvious decrease after 

 the optimum is reached. The optimum, however, is much higher in GOD- 

 LEwsKi's results than in those of Kreussler. 



Glyceria spedabilis (Expt. 15). 



Carbon-dioxide Carbon-dioxide 



in air. decomposed. 



3-1 % 2.10 



7-0 % 4-73 



io-4 % 5-75 



13-9 % 2.27 



Neriunt (Expt. 24). 



Carbon-dioxide Carbon-dioxide 



in air. decomposed. 



3-6 % 4-31 



132 % 3-62 



i8-5 % 3-23 



28-2 % 2.42 



[A research by Pantanelli (1904) explains to some extent why different 

 investigators have arrived at such varied optima for the proportion of carbon- 

 dioxide present in the air. Pantanelli shows that it really varies with the 

 intensity of light ; it is not apparent, however, why o-i per cent, of carbon- 

 dioxide in the air should be so injurious to plants, as had been affirmed to be 

 the case by Brown and Escombe (1902).] 



It is not surprising that greater concentrations of carbon-dioxide should 

 have an unfavourable influence on assimilation, since all vital processes are 

 inhibited by that gas (Lopriore, 1895). 



Let us turn now to the other question suggested at the end of the previous 

 lecture, viz. how does the carbon-dioxide reach the assimilating cells of the 

 leaf ? Owing to the different external conditions under which aquatic plants 

 live, the method adopted in their case cannot be the same as that existing 

 in land plants. In the former case the carbon-dioxide dissolved in water 

 must pass through the epidermal cells and may reach the other tissues either 

 while still in the dissolved state, or it may escape as a free gas into the inter- 

 cellular spaces, and by their means be distributed through the plant. The 

 cuticle of aquatic plants presents no more impediment to the through passage 

 of dissolved carbon-dioxide than it does to water, the medium of solution. 

 The conditions are quite otherwise in the case of the cuticle of land plants, 

 more especially that of the leaves. Carbon-dioxide gas is capable of penetrating 

 such cuticles, even although they be quite impermeable to water, just as carbon- 

 dioxide can diffuse through a layer of oil though water cannot. All the same, 

 the partial pressure of carbon-dioxide in the atmosphere is so small that the 

 amount which is able to pass through the cuticle is extraordinarily minute. 

 Were there no other means of entrance available than by diffusion through the 

 cuticle, practically no assimilation could take place under ordinary conditions. 

 On the other hand, Boussingault (1868) and Blackman (1895) established the 

 fact that formation of starch took place in such leaves as had obtained their 

 necessary supplies only by way of the cuticle from air rich in carbon-dioxide. 



It follows from this that in nature all highly organized leaves must be 

 provided with carbon-dioxide in some other manner. The stomata at once 

 suggest themselves as the portals for entrance of carbon-dioxide. Once the gas 

 has reached the intercellular spaces by their means, it can easily penetrate into 

 the individual cells after it has first become dissolved in the water of imbibition 

 present in the cell-walls. The amount of carbon-assimilation will thus depend 

 upon the number and distribution of the stomata and on the size of their 

 openings. Stahl (1894) and Meissner (1894) have demonstrated this conclu- 

 sively, for if a layer of vaseline be spread over leaves that bear stomata only on 

 their undersides, any formation of starch in the leaves is prevented. At low 

 temperatures leaves may be treated in this way without suffering any injury. 



