452 CONSTRUCTIVE AND DESTRUCTU'E METABOLISM 



substances which may or may not form integral components of protoplasm. 

 The living protoplast forms a cell-wall usually composed of cellulose but 

 sometimes of bodies resembling chitin, and this enclosing membrane 

 frequently undergoes subsequent chemical metamorphosis (Sect. 84). Every 

 soluble substance present in the cell exercises its appropriate osmotic 

 activity, but for the most part organic acids seem to be employed in the 

 maintenance and regulation of turgor (Sect. 24). The mere presence of such 

 bodies within the cell suffices to attain this end, whereas the protoplast 

 is able to grow only by assimilating appropriate food-material. The materials 

 of which the actual skeletal framework is composed are characterized by 

 a certain stability, whereas the plastic food-substances are liable to marked 

 and far-reaching changes, such as even the stored reserve food-substances 

 ultimately undergo. 



Experiments made with fungi show that widely different bodies 

 may serve as plastic material, and that each plant has its own specific 

 power of utilizing various food-substances (Sect. 66). Phanerogams appear 

 to be confined to a more restricted range of food-materials, although owing 

 to the complications introduced by any disturbance of the normal conditions 

 of nutrition it is difficult to determine the actual limitations of autotrophic 

 plants. The products formed by the plant itself do not form any criterion 

 as to the materials which it will require as food, and fungi are even able to 

 make use of many substances which they do not encounter in nature and 

 which are not formed in metabolism. Omnivorous fungi appear to have in 

 general more marked assimilative powers than higher plants, although 

 the power of synthesizing proteids possessed by the latter is absent from 

 certain of the former. 



In autotrophic plants all carbon compounds are ultimately derived 

 from some carbohydrate product of the assimilation of carbon dioxide, and 

 the nitrogenous and non-nitrogenous reserve food-materials are produced 

 by similar metabolic activity. The chief non-nitrogenous reserve-materials 

 are various soluble and insoluble carbohydrates (sugars, starch, cellulose, &c.), 

 fats, and in certain cases organic acids as well. Many of these substances 

 are widely distributed, whereas others are found only in certain plants, but 

 none of them appear to be indispensable, for in both storage and transloca- 

 tion various bodies may replace one another in the same or in different 

 plants. Thus oil is present in the seeds of many plants, starch in those 

 of others, while oil has occasionally been observed in the seeds of certain 

 grasses which normally contain starch 1 . Similarly various plants are able 

 to form starch or sugar from oil, or vice versa, and hence it is easy to. 

 understand why different reserve-materials may be stored up in the different 



1 Nageli, Die Starkekorner, 1858, p. 536; Pfeffer, Jahrb. f. wiss. Bot., 1872, Bd. vin, p. 490. 



