3^4 



LECTURE XX. 



quite so simple. It depends here no longer merely upon various transpositions of the 

 atoms of carbon, hydrogen, and oxygen in the molecules, as in the case of the carbo- 

 hydrates and fats : the proteid substances from which protoplasm is formed contain, 

 besides these elements, also a considerable quantity (about 1 5%) of nitrogen, and a 

 small quantity (about lo/o) of sulphur. It is obvious that if proteid substances are 

 to be derived from the first product of assimilation, starch, this can only take place 

 in so far as, in the course of metabolism, carbo-hydrates, or perhaps also fats, afford 

 the one constituent of proteid matters, while the nitrogen and sulphur are derived 

 from other compounds which the plant has taken up through the roots. It is certain 

 from what has been said concerning the nutritive materials of plants, that the nitro- 

 gen of the proteid substances is entirely derived from salts of nitric acid, in the more 

 highly organised plants at least ; while the sulphur is provided by salts of sulphuric 

 acid. It is essential, therefore, in order that proteid substances may arise in the plant, 

 that, on the one hand, starch is produced in the chlorophyll, and that, on the other, 

 salts of nitric and sulphuric acid are taken up by the roots. These ingredients must 

 somewhere and in some manner come together in the plant : chemical decomposi- 

 tions must take place in which, above all, the salts mentioned yield up their bases, so 

 that the nitrogen and sulphur, after certain unknown processes, may finally come into 

 combination with the elements of a carbo-hydrate or fat in order to produce a proteid. 

 Unfortunately, organic chemistry here leaves us unaided, so far as regards the question 

 of the chemical processes in the formation of proteids from the ingredients mentioned 

 above. The accuracy of our deduction is by no means enfeebled by this, however ; 

 and we will now see how far we can proceed by purely physiological methods. 



It is, in the first place, a very important question whether the whole of the proteid 

 substances of an ordinary plant is or is not produced during the process of assimila- 

 tion in the cells containing chlorophyll : at any rate, sulphates and nitrates can 

 penetrate as far as the green leaves, and there is scarcely anything absurd a pri'orz in 

 the idea that the formation of proteids perhaps begins with their aid, even during 

 the process of assimilation in the chlorophyll. But we have certain facts which show 

 that, although in general only certain non-nitrogenous organic compounds, and espe- 

 cially sugar, exist in them, even cells devoid of chlorophyll, and which therefore do 

 not assimilate, are in a position to produce proteid substances on the addition of 

 nitrates and sulphates. This proof Pasteur aff"orded by nourishing yeast-cells with 

 sugar and the salts named ; an experiment which may be easily repeated, and which 

 yields the result that by this means a few yeast-cells give rise to millions of new ones, 

 each of which is filled with living protoplasm. This is simply possible from the fact 

 that the yeast-cells are able to produce proteid substances from the materials — sugar 

 and salts of nitric and sulphuric acids — off"ered to them. If Yeast and other Fungi 

 are able to do this, however, there is no ground for assuming that cells devoid of 

 chlorophyll in the highly organised plants are not also able to produce proteid sub- 

 stances, taking the ingredients for this from their own carbo-hydrates, and from the 

 salts above mentioned which are absorbed by the roots. On the other hand, again, 

 this does not prove that in special cases proteid compounds may not also arise in cells 

 containing chlorophyll. The so-called unicellular Algse, the Palmellaceae, Conjugatese, 

 etc., consist indeed only of cells containing chlorophyll, and these, so long as they 

 vegetate, increase the quantity of their protoplasm (that is, their proteids) in pro- 



