Trelease — Botany During the 19th Century. 133 



converted into chemical or physical energy, which, under 

 the guidance of the living protoplasm of the cells, is utilized 

 for the breaking down of carbon dioxide and water, their 

 elements being then recombined into the organic products 

 referred to, the most usually recognizable of which is starch. 

 An attendant liberation of oxygen, constituting, with the ab- 

 straction of carbon dioxide, a purification of the air, so far 

 as the needs of animals are concerned, was made known 

 shortly before the century began, but it is to Saussure, at its 

 very beginning, that the connection of this with actual plant 

 nutrition is due, and it was he, too, who gave the first 

 clear demonstration that the remainder of plant food is de- 

 rived from the soil. A detailed study of this subject, as 

 well as of the metabolism or elaboration and transmutation that 

 food undergoes in the plant in its various nutritive and storage 

 processes, occupied particularly Sachs during the third quarter, 

 and Pfeffer during the last quarter of the century, Pfeffer's 

 ingenious investigation of the osmotic action of root hairs being 

 particularly interesting in connection with the physical prob- 

 lems of the absorption of crude materials and the retention of 

 organic products in the self -same organ. The last half of the 

 century has also produced the demonstration, on a large scale 

 in the field experiments of Gilbert and Lawes,and on a smaller 

 scale, but under more rigid control, in the laboratories of nu- 

 merous investigators, of the fact that while free atmospheric 

 nitrogen is not available for the nutrition of higher plants^ 

 which therefore as a rule require for their proper support an 

 abundance of available nitrogen supplied to the roots in the 

 form of nitrates, nitrites, etc., the Leguminosae as a class 

 make use of large, quantities of this atmospheric nitrogen, 

 not, indeed, in its free form directly, but through the inter- 

 vention of certain of the lowest fungi which inhabit their 

 roots as parasites, but, having the power of assimilating 

 nitrogen in forms in which it is not usable by the higher 

 plants, contribute to the latter enough of the product of their 

 own activity to more than compensate for whatever injury 

 they may cause by their parasitic invasion of the tissues of 

 the host. Indeed, pure cultures of these pseudo-parasites 

 are on the market, under the name of nitragin, for the inoc- 



