THE ASSIMILATION OF NITROGEN IN AUTOTROPHIC PLANTS 141 



potatoes, and wheat may be taken as examples of cultivated plants which 

 contain large quantities of nitrates. In the last two the nitrate amounts to 

 from i'5 to 2-8 per cent, of the dry weight. Even greater quantities (15 per 

 cent.) occur in Amarantus, to which may be added a whole series of weeds 

 such as Chenopodium, Urtica, &c. The maximum of nitrate is found in the 

 root, less in the stem and leaf, none at all in the seed. The nitrate increases as 

 the flowering period approaches, and decreases when fruiting takes place. 

 Frank (1888) has shown that these plants contain nitrates only when they 

 are able to absorb it by the root ; if they be grown in nutritive solutions 

 containing no nitrogen, or only ammonia, nitrates are entirely absent from 

 them. Hence we may conclude that the nitrate is not formed in the plant, 

 as Berthelot and Andre (1884) thought, but that it is absorbed from 

 without and stored for future use. Such storing of nitrate, however, is by no 

 means universal ; many plants absorb no more than they absolutely require. 

 The nitric acid is finally employed for the most part in the construction of 

 proteid, and to this end the combination of nitrogenous and carbonaceous 

 substances is especially necessary. We are accustomed to regard the carbo- 

 hydrates as the material source of the carbon in proteid, but it can scarcely be 

 doubted that other organic substances also, especially benzol derivatives, may 

 serve this purpose. We are as yet quite ignorant as to what is the first product 

 of union of the nitrogenous and carbonaceous substances. Treub (1895) 

 attempted to show that hydrocyanic acid was the first assimilation product 

 in Pangium edule, but the proofs he has given in this case do not appear to us 

 valid, and an extension of his hypothesis to other plants is scarcely justifiable. 

 [Treub (1905), in a more recent research, endeavours to show that hydrocyanic 

 acid may be the first assimilation product of nitrogen-containing material. 

 Numerous and interesting as the experimental data are which Treub has 

 brought forward, they are all in accord with the belief that the hydrocyanic 

 acid is a decomposition product of metabolism.] 



No definite answer can as yet be given to the question as to where the 

 assimilation of nitrates and the construction of proteid takes place, though 

 one is, generally speaking, inclined to hold the view that all plant cells may 

 be seats of proteid synthesis. Many authorities hold that most of the proteid 

 originates in the foliage leaves ; Schimper (1888, 1890), indeed, has ex- 

 pressed it as his opinion that nitrogen assimilation, like carbon assimilation, 

 is dependent on chlorophyll and sunlight. 



We may quote the following experiments in which he aimed at determining 

 this point (1888). The leaves of Pelargonium zonale are known to contain 

 generally an unusually large quantity of nitrate, and the amount present may 

 be further increased by keeping the plant in the dark or in moderate light ; it 

 disappears, however, in strong light in a few days. Those parts of the leaf 

 which contain no chlorophyll, such as occur in certain cultivated species of 

 Pelargonium, exhibit no alteration of the nitrate they contain on exposure to 

 light, and the same is true of the aerial roots of Tradescantia selloi. Schimper 

 also inquired into the origin of the large amounts of calcium oxalate to be found 

 in illuminated leaves which were provided with calcium nitrate, and he sup- 

 posed that the oxalic acid produced in the course of metabolism took the 

 place of the nitric acid and united with the lime. (As to this supposed 

 function of oxalic acid, compare Benecke, Botan. Ztg. 1903 ; the subject 

 will be again referred to in Lecture XVI.) There can scarcely be any doubt 

 whatever that a vigorous synthesis of proteid takes place in leaves which are 

 strongly illuminated, but the influence of sunlight and chlorophyll can only be 

 indirect ; their influence depends on the fact that carbohydrates are present 

 during carbon assimilation in larger quantities in the region of origin than in 

 other regions to which they must first be transferred ; further, then, chemical 

 construction may be more advantageously effected in carbon-assimilating cells 



