150 LECTURE IX. 



genous organic substances of less complex composition are 

 formed as intermediate products. From the researches of 

 Biltz, of Kellner, of Emmerling, and of Borodin, it appears 

 that crystallisable nitrogenous organic substances, such as 

 asparagin, leucin, and others (we may, for the sake of brevity, 

 term them amides] are frequently to be found in leaves. It is 

 quite possible, as Kellner and Emmerling have suggested, 

 that these substances may be formed synthetically in the leaf. 

 We may imagine the processes to be somewhat as follows. 

 Since all these substances contain nitrogen in the form of 

 ammonia, the first step would be the formation of ammonia 

 or ammonia compounds from the nitric acid absorbed as 

 nitrates by the root ; that some process of this kind actually 

 takes place is suggested by Emmerling's observation that no 

 nitrates are usually present in the leaf, by the fact mentioned 

 in a previous lecture (p. 127) that certain plants are unable to 

 assimilate the nitrates which they absorb, and by Hosaeus' 

 observation that ammonia salts were to be found in a number 

 of different plants which he analysed, even when they had 

 been supplied with manure which did not contain ammonia. 

 The ammonia thus formed combines with formic aldehyde or 

 one of its polymers to form one or other of these amides, and 

 this combines with some form of non-nitrogenous organic 

 substance and with sulphur to form proteid. This view of 

 the mode of formation of proteid is supported by the fact, 

 mentioned in a previous lecture (p. 124), that even green 

 plants can take up their nitrogen in the form of amides. 



But we must not overlook the fact that amides, and probably 

 also ammonia, are produced in the plant by an analytic pro- 

 cess, by the decomposition of proteid, and it must be admitted 

 that they may be formed in this way in the leaf. The condi- 

 tions which determine this process have been made evident by 

 the observations of Borodin. He found that no asparagin 

 could be detected in shoots of certain plants when they were 

 growing under normal conditions, but that the appearance of 

 asparagin in considerable quantity could be induced by cutting 

 off the shoots and keeping them for some time in the dark. 

 The explanation of this fact is doubtless this, that the proteids 



