Osmotic Pressure in Plants. 159 



Many of the dune plants tested by Fitting possessed an 

 osmotic pressure markedly lower than that of the same forms 

 in the mountains and boulder desert. Iwtting concludes, 



■'The lower pressure upon the sand seems to indicate that ths 

 water suppl\- in the sand is less limited than in other dry desert 

 soils."' (]). 2.^1). He also calls attention to the low resistance 

 offered by sand to abso&ption by roots, and cites the rapid 

 downward penetration of rainwater in such soil and to the 

 possi!)ility of extensive root growth therein. 



Fitting repeatedly asserts that the power to develop such 

 high pressures as he has demonstrated is a purposeful adapta- 

 tion for living in dry soils! * 



In phvsical terms, ij wc assume that the movement of water 

 from soil to plant depends upon the gradient of diffiusion ten- 

 sion which exists between the soil films, on the one hand, anJ 

 the solutions within the roots, on the other; it follows that the 

 lower the internal diffusion tension, the drier will be the soil 

 wdien e:iuilibrium is reached. \'apor pressure is proportional to 

 diffusion tension (is, indeed, a special case of this) and we have 

 merely to consider that the vapor pressure of the internal solu- 

 tions decreases as their osmotic concentration rises, while the 

 vapor tension of the soil moisture films decreases rapidly as 

 as tlie films becomes more tenuous. Thus we have an attrac- 

 ti( n (f the cell sap of roots, for water, opposed by an attrac- 

 tion of the s:)il, the former force depending upon the osmotic 

 c oncentration of the sap and the latter upon the thickness 

 and arrangement, as well as upon the osmotic concentra- 

 tion ff the soil moisture films. It is from these con- 



arrested by being moistened and thus a dtinc is built up. (See MacDougal. I). T., Publi 

 cation Of of the Carnegie Institution. 19()S. p. .^O) Such a dune may attain a hcii^ht of sev 

 era! miters, being miinly held in place by stem? and roots of thrifty plants, which derive 

 moisture from below and suggest to the eye the presence of a continuous supply of water. 

 Dunes of this sort, well covered with vcgct^'.tion. often occur where the sf)il is apt to be 

 moi^t for the longest perio:Is. as along the niargin of an out cropping drainage stratum, and 

 the accumulated sand shouUl act as a very perfect mulch to conserve the natural soil mois 

 ture of such localities. Tnus.tho more vigorous plant growth here m ly be largalv due to the 

 sand mulch \\'hethor or not th^se considerations may have any bearing upo.n the nature 

 of the dunes and dune vegetation in the vicinity of Biskra I am unable to surmise. That 

 th?re are out croppi ig drainage strata in that vinicity is clear from the existence of the 

 original ori. itse'.f. the spring; of Hammam Salahin. etc. 



*''.Manwird dice Befahgung als jiussert zweckmassig betr.ichten miissen, um auf 

 : ehr trockcnem Ho ien gedeihen zu. koanen." (p246) To m\- mind nothing at all is gained 

 and the general muddle of ecology is greatly augumented by such methods af statemeat 

 It were fully as well to anthropomorphose the soil as well as the plant, and to hold that 

 extreme dryness is a svtl adaptation by which the Sahara soil better succeeds in preserving 

 itself largely untrammelled by vegetation! 



