THE CONVERSION OF THE PRODUCTS OF ASSIMILATION. Ill 175 



of such amides ; certain products arising during carbon assimilation are necessary 

 for proteid formation. We must again take into consideration the carbo- 

 hydrates in this relation, all the more so since if these are supplied in sufficient 

 quantity proteid reformation can take place in the dark (compare p. 144). The 

 establishment of this fact is far, however, from solving the chemical problem 

 of the synthesis of proteid from glucose and asparagin. The glucose at all 

 events must undergo a fundamental change during this process, since it is incon- 

 ceivable that it can be introduced into the proteid molecule exclusively as 

 a carbohydrate group ; indeed, it is by no means certain that such a group 

 occurs in vegetable proteid. Since, however, in animals carbohydrates can be 

 formed from proteid, we must admit the reverse process to be possible in the 

 plant. 



In addition to the regeneration of proteid out of the products of its de- 

 composition we must glance finally at the construction of fats, which we have 

 seen in our last lecture to be present as a reserve material in seeds. Fat also 

 occurs in the vegetative organs, and, according to certain authorities, it may be 

 supposed to travel in these organs either as fat or, after preliminary decom- 

 position into glycerine and a fatty acid. No one can believe, however, that 

 the entire mass of fat occurring in a seed could have entered it in that form. 

 On the other hand, the same materials may be seen travelling to oily seeds as 

 to those poor in oil, i. e. carbohydrate or, e. g., mannite in the olive, a substance 

 which takes the place of carbohydrate there. Again, in all oily seeds (PFEFFER, 

 1872) in the young condition large quantities of starch occur, which, when the 

 seeds are ripe, is replaced by a fatty oil. This does not appear to be effected by 

 the respiration of starch and its change into water and carbon-dioxide while fat 

 wanders in to take its place; on the other hand, the fat must be derived from the 

 starch, for one can demonstrate its appearance in isolated unripe seeds into 

 which no entry of fat is possible. Just as we saw earlier that an alteration 

 of fat into carbohydrate took place during the germination of seeds, so now 

 we may note that in ripe seeds starch is changed into fat. From the chemical 

 point of view this change is extraordinarily difficult to appreciate, i. e. the origin 

 of a substance poor in oxygen from one relatively rich in that element. A 

 chemical reaction of this type has not as yet been observed externally to the 

 cell. We must therefore content ourselves for the present with the observation 

 of the fact without being able to enter more deeply into the meaning of the 

 phenomenon. It must be noted, however, that the alteration of starch into fat 

 is not limited to seeds. In trees also starch is, during winter, at least partly, 

 altered into fat, and in spring starch is again reformed from fat. [According to 

 NIKLEWSKI (1905), the relationship between sugar and fat cannot be explained 

 from the chemical point of view and physiologically also it is very doubtful.] 

 Both these processes depend in a variable manner on temperature (A. FISCHER, 

 1890) ; low temperatures tend to induce the formation of fat, high temperatures, 

 starch. Hence one can in the middle of winter bring about a reformation 

 of starch in amputated twigs by bringing them into a warm room. The 

 significance of this phenomenon is still a great puzzle, and its physiological 

 reasons are also but little understood. An increase in dissolution of starch 

 accompanying a decrease in temperature is known to be independent of the 

 formation of fat ; in the potato, for example, the ' sweetening ' depends, at 

 temperatures just above o C., on the formation of sugar out of starch 

 (MiJLLER-THURGAU, 1882), which is promoted by quite low temperatures. This 

 disappearance of starch at low temperatures cannot be accounted for by special 

 peculiarities of diastase. 



We have now obtained a knowledge of the changes which certain organic 

 compounds undergo in the green plant ; at the same time, we have glanced 

 at only a relatively limited number of chemical substances, namely, the proteid 

 bodies and the crystalline nitrogenous organic substances resulting from their de- 



