2l8 LECTURE XII. 



when a cell-wall becomes much thickened, that is, when 

 many layers of cellulose are found, the whole of the proto- 

 plasmic contents of the cell are gradually used up in the 

 process : and further, that, as has already been stated (pp. 

 151, 177) no non-nitrogenous plastic substances can be de- 

 tected in growing- points. 



It appears, then, that in these cases the non-nitrogenous 

 organic substance, either formed from carbon dioxide and 

 water or absorbed as food, is not directly converted into 

 starch or cellulose, but goes in the first instance to build up 

 protoplasm, and that it is in consequence of the decom- 

 position of protoplasm that these carbohydrates make their 

 appearance : and we may, perhaps, go so far as to infer that 

 all the reserve-carbohydrates are produced in this way. 



With regard to the fats, it is generally assumed that 

 they are formed directly from carbohydrates, because as the 

 fats increase in quantity the carbohydrates diminish. For 

 example, oily seeds contain starch whilst they are unripe, 

 but as they ripen the starch disappears and is replaced by 

 fat. 



The first objection to this view is the obvious one that 

 the observed fact does not prove the direct conversion of 

 carbohydrate into fat. And further, we know of no means 

 by which such a conversion could be effected. Hoppe- 

 Seyler, in his investigations on this subject, has found that 

 when carbohydrates are heated with caustic potash, or when 

 they are caused to undergo putrefaction, they yield a series 

 of fatty acids, and he infers that the fats found in animals 

 and plants are derived directly from carbohydrate. But the 

 acids formed in his experiments are only the lower members 

 of the series, and not those which enter into the composition 

 of the more common fats: moreover, he does not suggest 

 how the necessary glycerin is formed from carbohydrate. 



The evidence in favour of the view that fats are derived 

 from the protoplasmic cell-contents is of a more satisfactory 

 character. In studying the effects of starvation upon the 

 cells of plants, Cunningham found, in the case of certain 

 Fungi, that if the spores be cultivated in distilled water, the 



