ROOT HAIRS: ABSORPTION FORCES 49 



According to measurements of Schwarz (1883), root 

 hairs produced in moist air reach lengths of 0*5 to 3 mm. : in 

 earth the length is usually less ; in water it is more. They 

 may occur to the number of 200 to 400 per sq. mm. of root 

 surface, and the increase of absorbing surface over that of a 

 hairless root may be as much as eighteen times. 



Many plants when grown in water produce no root hairs ; 

 this may well be due to reduction of the oxygen supply 

 below the minimum necessary to the very active growth 

 entailed in their production. The same lack of root hairs 

 is seen in new roots produced by plants growing in soil 

 saturated with nitrogen instead of air. Many aquatic 

 plants, however, possess root hairs. In Hydrocharis (Arber) 

 they are remarkably long, in Elodea they are produced only 

 when the root enters the mud, in Lemna they are absent. 

 In some marsh plants, such as Myrica Gale, no root hairs 

 are formed. They are, as a rule, absent from mycorhizal 

 roots. 



Forces of Absorption. — The actual absorption of the 

 water depends on the osmotic pressure of the cell sap of the 

 root hair. The direction of the flow is determined by the 

 osmotic gradient ; water passes towards the solution with 

 the higher osmotic pressure. Normally, of course, the 

 vacuolar fluid of a plant cell, containing in solution small 

 quantities of salts together with larger quantities of organic 

 crystalloids such as sugar, has a very considerable osmotic 

 pressure. We may take 10 atmospheres as a conventional 

 figure, and this is roughly the pressure exercised by a 15 per 

 cent, solution of cane sugar. As the concentration of the 

 soil solution is a very small fraction of this, the mechanism of 

 transference of water from soil to plant is provided for, at 

 least when the soil contains abundant water. When the 

 water supply runs low, however, complications arise. 



In the first place it must be pointed out that the whole of 

 the osmotic pressure need not be employed in drawing water 

 into the cell. It is a familiar fact that, when a cell is plenti- 

 fully supplied with water, hydrostatic pressure acts against 

 the elastic pressure of the cell wall and distends it ; this is 



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