CHEMICAL AND PHYSIOLOGICAL STUDIES. 173 



the way up the root, aud, consequently, the osmotic balance of the 

 hair is being as constantly disturbed, so tliat fresh water and salts 

 are drawn in again from the soil. The turgid condition of a cell 

 when the upper limit of osmosis has been reached determines a 

 certain stretching of the cell-wall (turgidity), and this stretching 

 is a great aid to the growth of the wall in area. 



Solutions may be prepared which are said to be isotonic — 

 that is, when a cell is placed in them neither influx nor exit of 

 water takes place. But a cell, such as a root-hair, when placed 

 in a solution which is ever so slightly greater in concentration 

 than the sap in the hair, will sufler a certain amount of 

 exosmosis — that is, water will pass out, and tlie hair will 

 shrink (see Plasmolysis, Chap. ii.). 



In speaking of selective absorption by the root-hairs, it was 

 shown that certain salts may be taken in to the exclusion of 

 others in the soil ; this is quite true, but, nevertheless, the 

 factors determining this selective absorption, depend not only 

 upon the regulating influence of the ectoplasm, but also upon the 

 physical nature of the salt in solution in the sap inside the hair — 

 that is, the osmotic activity of the substances in the cell must be 

 considered in addition to the selective influence of the ectoplasm. 



In addition to these few statements with regard to osmosis 

 and turgidity, it must be mentioned that during the distribution 

 of the assimilable food in a plant, the question of the osmotic 

 nature of the substances in the cells to which this food passes, is 

 a highly important one, and is, moreover, one which chiefly 

 determines whether or no such a substance as sugar, for instance, 

 shall be taken into any given cell. The fact that the surplus of 

 the circulating food is converted into reserve food, leads to a 

 constant movement of the diff'usible materials towards the 

 storing cells (see also Appendix at end of Chap x.). 



With regard to the absorption of nitrites and nitrates by 

 the root-hairs, it is an interesting fact that certain Bacteria 

 exist in the soil which are capable of converting ammonia and 

 free nitrogen into nitrites and nitrates, Several species of 

 Bacteria probably exist, each one taking on a single stage in 

 this process. In the Leguminosee there are certain tubercles to 

 be found on the rootlets, and these tubercles have been shown to 

 be composed of dense masses of Bacteria {B. radicicola) belonging 

 to a species which is able to convert the free nitrogen of the 



