220 PHYSIOLOGY 



Considerable light is thrown on the subject of oil-formation by the results 

 ,of recent researches on the nutrition of algae and fungi. Beijerinck' made 

 cdrti^rative cul't'iires of diatoms taken from the soil, and he found that so 

 long as culture conditions were favourable, any fat that might be formed 

 was at once assimilated. If, however, some adverse influence checked the 

 growth of the organism while carbonic acid assimilation was in full vigour, 

 fat was at once accumulated. The adverse influence in this case was the 

 lack of nitrogen, and Beijerinck considers it an almost universal rule in plants 

 and animals, that where there is absence of nitrogen, in a culture otherwise 

 suitable, fat-oils will be massed in those cells which are capable of forming 

 oil. He observed that in two of the cultures of diatoms the one which alone 

 was supplied with nitrogen grew normally, while the other, deprived of 

 nitrogen, formed quantities of oil-drops. Wehmer^recordsthe same experience 

 in his cultural study of Aspergillus. Sphaeroid fat-cells, similar to those 

 described by Zukal in calcicolous lichens, were formed in the hyphae of a 

 culture containing an overplus of calcium carbonate, and he judged, entirely 

 on morphological grounds, that these were not of the nature of reserve-storage 

 cells. 



StaheP has definitely established the same results in cultures of other 

 filamentous fungi. In an artificial culture medium in which nitrogen was 

 almost wholly absent, the cells of the mycelium seemed to be entirely 

 occupied by oil-drops, and this fatty condition he considered to be a symptom 

 of degeneration due to the lack of nitrogen. These experiments enable us 

 to understand how the hyphae of calcicolous lichens, buried deep in the 

 substratum, deprived of nitrogen and overweighted with carbonic acid, may 

 suffer from fatty degeneration as shown by the formation of " sphaeroid-cells." 

 The connection between cause and effect is more obscure in the case of 

 lichens growing on the surface of the soil, such as Baeomyces roseus, or of 

 tree lichens such as the brown Parmeliae, but the same influence — lack of 

 sufficient nitrogenous food — may be at work in those as well as in the endo- 

 lithic species, though to a less marked extent. 



It seems probable that the capacity to form oil- or fat-cells has become 

 part of the inherited development of certain lichen species and pei'sists 

 through changes of habitat as exemplified in Lecanora calcarea\ 



In considering the question of the formation and the function of fat in 

 plant cells, it must be remembered that the service rendered to the life of 

 the organism by this substance is a very variable one. In the higher plants 

 (in seeds, etc.) fat undoubtedly functions in the same way as starch and 

 other carbohydrates as a reserve food. It is evidently not so in lichens, and 

 in one of his early researches, Pfeff'er^ proved that similarly oil was only 



^ Beijerinck 1904. ^ Wehmer 1891. ^ stahel 191 1. * See p. 218. 



5 Pfeffer 1877. 



