ALTERATIONS WHICH THE NUTRIMENT UNDERGOES. 549 



only a small fraction is employed to replace the cell 

 substance which has been destroyed, and that the greater 

 part remains in the juices of the cell, and is subjected to 

 the decomposing action of the protoplasm while it is in con- 

 tact with it in a soluble form ; finally, a very varying por- 

 tion is employed for the formation of new cell material, 

 and thus meets the demand for growth and multiplication. 



A more accurate insight into the quantitative division 

 of these respective roles is however impossible at the 

 present time. It is, indeed, frequently doubtful whether 

 a body which is found by analysis to be a constituent part 

 of the organism should be looked on as a plastic material 

 suitable for its future functions, or only as an excretory 

 product. It sometimes happens that substances which 

 are split up from more complex compounds by intra- 

 molecular respiration and excreted by the cells, can still 

 act as nutrient materials, and can be employed by other 

 or it may be the same cells for the formation of plastic 

 material ; and in this case it is more or less a matter of 

 choice whether these bodies are reckoned among the 

 excreta or among the plastic materials. This difficulty, 

 however, is present to a much less degree in the lower 

 fungi than in the higher plants; for in the former we 

 may look on the gaseous bodies, such as carbonic acid, 

 with certainty as excreta, whilst the higher plants can 

 assimilate them again. 



As nitrogenous plastic materials, we have chiefly the Nitrogenous 

 whole group of proteid bodies ; these are present in materials, 

 solution in the juices of the cell, and then either undergo 

 decomposition in the protoplasm, forming for example 

 new cell substance, or are deposited in an insoluble 

 condition in the cell protoplasm. Nageli was able to 

 make out the surprising fact that yeast cells excrete 

 albumen and peptone; the peptone being produced in 

 non-fermenting neutral or acid nutrient media, and 

 albumen in fermenting or non-fermenting fluids with an 

 alkaline reaction. In addition to the proteid substances 

 numerous amides and amido-acids, especially asparagine 

 and glutamine, are found in the higher plants. These 

 may be looked on partly as forerunners, and partly as the 



