THE PLANT. 



carbonate of potash and ammonia, which we certainly 

 know to be nutritive substances, act upon many plants 

 as poison, even when present in th'e water which circu- 

 lates 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 impene- 

 trable for salt solutions.* 



Those mineral substances which, like iron, are con- 

 stant constituents of all plants, though present only in 

 very small proportions, must be regarded very different- 

 ly from those metals which Forchhammer found in 

 woody plants. 



We know the part which iron performs in the ani- 

 mal organism, in which it is present in comparatively 

 no larger quantities than in the seeds of cereals ; and 

 we are fully convinced that, without a certain amount 

 of iron in the food of animals, the formation of the 

 blood corpuscles, the agents of one of the chief func- 

 tions of the blood, is impossible. Hence, by the law 

 of dependence, which links together the life of animals 

 and plants, we are compelled to ascribe to the iron in 

 the plant also an active part in its vital functions so 

 material that the absence of that metal would endanger 

 the very existence of the plant. 



Hitherto chemistry has attributed a positive part in 

 the vital process of plants to those incombustible sub- 

 stances only which are common to all, and which differ 

 only in the relative proportions in the plants. But 



* 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 with which the short limb is closed. 

 By the capillary action of the bladder, the water exuding in gaseous 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 pressure 

 upon the surfaces of both bladders, which forces the salt-water or the oil 

 through the bladder into the tube. (' Researches into some of the Causes 

 of the Motion of Fluids, by J. v. Liebig. Brunswick : Fr. Vie wig & Son. 

 1848.' p. 67.) A plant in similar conditions is just like a tube closed 

 with penetrable porous membranes. 



