560 CHEMISTRY IN AGRICULTURE AND PHYSIOLOGY. 
brane. It has long been decided that solid substances, 
insoluble in water, cannot pass into plants, but this may be 
doubtful of the colouring-matter of phytolacca, of decoction 
of logwood, of infusion of saffron, &c., since many obser¬ 
vers, e. g. De Candolle, have seen such colouring-matters 
pass into living plants. But all accurate observations indi¬ 
cate that this does not happen in uninjured roots, but only 
occurs when the coloured fluid comes in contact with wounds 
of the plant. 
Since the discovery of endosmose most vegetable physio¬ 
logists have assumed it as an axiom that the absorption of 
cells depends wholly and solely upon the laws of endosmose, 
none of the peculiar forces of the living cell co-operating. 
All the conditions to bring about good strong endosmose 
do really exist in the living vegetable cells, namely, a mem¬ 
brane freely penetratable by watery fluids ; on the one side of 
this the cell-sap, which contains proteine substances, dextrine, 
sugar, &c., in solution ; on the other side, the water occur¬ 
ring in nature, in the state of an extremely diluted saline 
solution. 
Since the leaves have a large surface with a comparatively 
small mass, they are fitted to evaporate a great quantity of 
water; thus, for example, in Hales’ experiment, a sun-flower, 
three and a half feet high, lost on an average a pound and 
fourteen ounces of water daily on warm and dry days. So 
considerable a loss of water cannot remain without reaction 
upon the absorption of the root-cell. For since the sap in 
the cells of the leaves becomes so much more concentrated, 
through the loss of water, their power of inducing endosmosis 
will increase in proportion ; they replace the water taken 
from them from the cells of the stem, and so this action is 
continued through the whole tissues of the plant, down to 
the roots, which strive to absorb water from without in the 
same proportion as it is evaporated from the leaves. A 
proof that the evaporation of the leaf actually increases the 
absorption is again furnished by the experiments of Hales, 
according to which the quantity of water that a shoot 
absorbs is in direct proportion to the number of its leaves; 
and the quantity of water absorbed sinks to one half, when 
half the leaves are cut off the shoot. 
The question, what nutrient matters serve for the food of 
the plants, includes a twofold one. First, what elementary 
materials are made use of by the plant, in the formation of 
its substances? and, second, what are the combinations in 
which these elementary materials are taken up by plants? 
The number of elementary substances which occur in 
