82. S. W. Johnson on some points of Agricultural Science. 
present in the soil, and thus the normal humidity of the struc- 
ture is preserved. But if the plant be situated in a close hot- 
house, or in a Ward’s case, the atmosphere of which is constant- 
ly saturated with aqueous vapor, there can be-no evaporation of 
water from the leaves, there can be no transpiration of water 
through the plant and no absorption of it by the roots, except 
to supply what becomes a solid constituent of the tissues or is 
decomposed in the nutritive process, The same is true of potash 
or any other substance held in solution in the soil-water. Asa 
result of this principle the land plant collects the potash, phos- 
phoric acid, silica, &c., needed for its organization, from the vastly 
dilute solutions of these bodies which form the water of wells or 
of the soil, just as the fucus gathers its iodine from the ocean, 
although the marvellously delicate reagents which we possess for’ 
iodine scarcely enable us to detect this substance even in highly 
concentrated sea-water. 
_ Says Gmelin, (Handbook of Chemistry, Cavendish Soc’s, ed., 
vol. 1, p. 248,) “the quantity of iodine contained in sea-water is 
so small that Tennant, Davy, Gaultier, Fyfe and Sarphati were 
not able to find it. Balard, however, found it in the water of 
soaaath part—Otto. 
The selecting power which is possessed by plants is fully ex- 
elias and defined y osmotic diffusion. Within certain easy 
imits the plant imbibes only those kinds of matter and those 
quantities, which it requires to develop its organism, and which 
diffuse into it in consequence of assimilation in the cells. These 
limits are not so narrow or inflexible as to make the finding of 
the conditions of growth impossible, and within them, the plant 
lives'and expands, but is itself influenced in its life and in the 
direction of its enlargement, by the quantities, absolute and rela- 
tive, of the nutritive or soluble matters, that happen to surroun 
it. Could we grow two plants in precisely identical conditions, 
we should find their composition alike in all their parts. Th 
variations in the composition and amount of the ash of plants 18. 
probably connected with the different relative development of 
he separate organs, and this again (in part) with the-relative 
quantities of food present in the soil water. Thus the ash of 
