LECTURE XX. 



CHEMICAL METAMORPHOSES OF THE PRODUCTS OF ASSIMILA- 

 TION. PHYSIOLOGICAL CLASSIFICATION OF THE PRODUCTS 

 OF METABOLISM. 



Starch being the sole first visible product of assimilation, it follows obviously 

 that all the other organic compounds of the plant must be produced from it by 

 chemical changes. This statement oflFers in fact no diflSculties even from the chemical 

 side, so far as the majority of the constituents of the plant are concerned, i.e. the 

 carbo-hydrates, of which cellulose, sugars, and inulin especially come into considera- 

 tion here. These substances are chemically so nearly allied to each other, that not 

 only starch and inulin, but even cellulose may be transformed by the simplest 

 chemical reactions into sugars ; and from the physiological processes it may be 

 demonstrated with certainty that starch and cellulose are produced from sugar and 

 inulin within the living plant. 



Besides these carbo-hydrates, however, the fats also play a prominent part in 

 the economy of the plant. Perhaps there is no living protoplasm which contains 

 no fat : but its physiological significance becomes much more evident in that the 

 great majority of ripe seeds contain large quantities of fat together with starch, or 

 very great quantities of fat without the latteri They may contain as much as 60% 

 and more of their weight of fat; and even the spores of very many Cryptogams 

 abound in it. That sugar and starch are formed during germination at the cost of 

 the fat which is accumulated in the seeds and then used up, I demonstrated in detail 

 in 1859 on a series of oily seeds ^; and it was already known that, before maturity, 

 such seeds contain no fat, but only starch and sugar. Such unripe seeds (e. g. of 

 PcBonia) may be detached from the mother-plant, and allowed to lie in moist air, with 

 the result that the starch disappears and is replaced by fatty oil. From such obser- 

 vations it follows with certainty that the fats of the plant may be produced from 

 carbo-hydrates, and that the latter are formed at their expense. It is practically 

 immaterial for us how far chemical fgrijujlas are able to afford information as to the 

 processes here taking place ; in any case we arrive at the result that the carbo- 

 hydrates, as well as the fats of the plant, derive their origin from the starch assimilated 

 in the chlorophyll-grains. 



The derivation of the third most important group of the materials of the plant, 

 viz. the proteid substances, from the original product of assimilation, does not appear 



* On the formation of starch during the germination of oily seeds, cp. Sachs' Bot. Zeitung, 

 1859, P- 178- 



Y 2 



