QUANTITATIVE SELECTIVE POWER 119 



According to the researches mentioned, the glycerine compounds of stearic, 

 palmitic, phloric and suberic acids are the most important impregnating substances. 

 Some of these compounds are insoluble in chloroform, ether, &c., but are saponified 

 by potash '. The double refraction of the cuticle is due to the presence of these 

 substances (Sect. 13), which have, it may be noticed, different melting-points. 



SECTION 22. Quantitative Selective Power. 



Every plant and every cell has definite specific needs, to satisfy which 

 certain substances are absorbed in large amount, while of others little or no 

 use is made. Thus most plants are unable to directly utilize atmospheric 

 nitrogen, nor is the carbonic acid gas of the air of any value to those 

 without chlorophyll, but all aerobic plants consume oxygen in large quantities. 

 In the same way the substances presented to a plant in a nutrient solution 

 are absorbed in direct proportion to the amounts used and required in 

 metabolism, so that a substance present in abundance may not be diminished 

 in quantity, while the last traces may be absorbed of another substance 

 present in small amount. Both higher and lower plants possess this selective 

 power, which is dependent upon and is regulated in a specific manner by 

 the vital activity of the plant itself, independently of whether the substances 

 absorbed are essential or non-essential to its well-being. 



It is quite immaterial whether the selected substances are employed as 

 building material or stored up as food reserves, or whether they are returned 

 to the external medium in changed form as waste metabolic products for 

 which the plant has no further use. Thus in a confined space a plant will 

 absorb the last traces of oxygen present, although the whole of this is finally 

 exhaled again in the form of carbonic acid ; while of the sugars which 

 a fungus or yeast-cell extracts from a culture fluid, large quantities are 

 returned to the surrounding media in the form of carbonic acid gas, oxalic 

 acid, and alcohol. 



This selective power is made manifest by changes taking place in the 

 composition of the surrounding medium, as well as by the retention and 

 accumulation of particular substances within the plant. In the latter case, 

 the results produced by selective action are well shown when the ash 

 constituents accumulated by the plant are contrasted in quality and amount 

 with the mineral constituents present in the nutrient solution with which 

 the plant is supplied. It can then be seen that from ordinary tap-water, 

 containing mere traces of salts, plants may collect large quantities of non- 

 volatilizable mineral constituents, and moreover that these constituents are 



life of cork cells, see Koppen, Das Verhalten d. Rinde unserer Laubbaume, 1889, p. 480 (Nova 

 Acta d. Leopoldin. Akad.). 



1 Gilson, Bot. Centralbl., 1891, Bd. XLV, p. in ; Fliickiger, ibid., 1892, Bd L, p. 90; \Visse- 

 lingh, ibid., 1895, Bd. LXII, p. 234. 



