56 THE PLANT. 



Hekth, show that the roots of land and water plants 

 absorb from very dilute saline solutions water and salt 

 in proportions entirely different from those in the fluid ; 

 in all cases a greater proportion of water, and a less 

 quantity of salt. In plants watered with very dilute 

 solutions of salts of baryta, Daubeny found no baryta, 

 whereas Knop in similar experiments detected this sub- 

 stance. The general result of all these experiments is 

 that, of themselves, the plants have not the power of 

 offering a permanent resistance to the chemical action of 

 salts and other inorganic compounds upon the exceedingly 

 fine membrane of the root. 



Most land-plants in their natural state in the soil can 

 bear no salt solutions, as concentrated as in these experi- 

 ments, without sickening and dying ; and even carbonate 

 of potash and ammonia, which we certainly know to be 

 nutritive substances, act upon many plants as poison, even 

 when present in the water which circulates in the ground 

 only in sufficient quantity to impart a blue tint to red 

 litmus paper. On the other hand, it would be very 

 wonderful if the roots of a plant outside the soil, and in 

 conditions not suitable to their nature, should, under the 

 influence of evaporation, be impenetrable for salt solu- 

 tions.* 



* If the long limb of a syphon -shaped tube, filled with water and 

 closed with thick pieces of pig or ox bladder tied over both openings, 

 is placed in salt-water or oil, and the other limb is exposed to the air, 

 the water evaporates in the pores of the bladder Avith which the short 

 limb is closed. By the capillary action of the bladder, the water 

 exuding in gaseoiis form is taken up again on the other side of the 

 bladder, and a vacuum is thus created in the interior of the tube, 

 whence there is an increased pressui-e upon the surfaces of both 

 bladders, which forces the salt-water or the oil through the bladder 

 into the tube. (' Eesearches into some of the Causes of the Motion 

 of Fluids, by J. v. Leibig. Brimswick : Fr. Viewig& Son. 18^8.' — 

 p. 67.) A plant in similar conditions is jiist Hke a tube closed Avith 

 penetrable porous membranes. 



