SPONTANEOUS movements in plants. 
835 
to the lateral walls. Artificial light produces the same effect 
as daylight. 
Analogous to the circulation of the protoplasm within the 
cell is that of the sap or nutritive fluid through the whole 
plant, passing through the permeable walls of the cells. This 
circulation of the sap, by which fluid is conveyed equally to 
all parts of the plant, apparently in opposition to the laws of 
gravity, is, no doubt, explicable to a certain extent by the appli¬ 
cation of known physical laws, of which the most important 
are capillary attraction, osmose, or the law by which a less 
dense fluid passes through a permeable diaphragm to mingle 
with a denser fluid, and the upw ard pumping force to supply 
the partial vacuum occasioned by the evaporation of water 
from the leaves. Allowing, however, full scope to all these 
physical forces, there would seem to be a residuum of energy 
still unaccounted for connected with the vitality of the plant 
itself. In particular, the selective pow er of plants in absorb¬ 
ing from the soil a larger portion of those ingredients w r hich 
are required for the formation or healthy life of their tissues, 
is an absolutely unexplained phenomenon. A familiar instance 
of this is furnished by the difference in the amount of silica 
absorbed by corn-crops and by leguminous plants, amounting 
in the former case to 2 5 per cent., in the latter to ’3 per cent, 
of the dry foliage. Indeed, if any two plants are grown 
together, side by side in the same soil, the constitution of 
the ash, that is, of the solid ingredients derived from the soil, 
whll be remarkably different; w 7 hile in the same plant in the 
same soil the constitution is constant. It was pointed out by 
the Duke of Argyll, when criticising Darwin’s 6 Origin of 
Species/ how unavoidable it seems, in describing the pheno¬ 
mena of nature, to use language involving the idea of con¬ 
trivance and design. In the same manner it seems impossible 
to describe the process of vegetative life without appearing to 
attribute to the plant some conscious power of its own. A 
striking instance of this, as well as of the liability to consider 
a mere statement of an obscure law 7 in other terms as an 
explanation of that law, occurs in an admirable treatise on the 
growth of plants—Johnson’s e How Crops Grow.’ * 66 The 
cereals are able to dispose of silica by giving it a place in the 
cuticular cells; the leguminous crops, on the other hand, 
camiot remove it from their juices; the latter remain saturated, 
and thus further diffusion of silica from without becomes 
* ‘ How Crops Grow; ’ A Treatise on the Chemical Compositions, 
Structure and Life, of the Plant, for Agricultural Students. By S. W. 
Johnson. Revised and adapted for English use by A. H. Church and W. 
T. T. Dyer. London : Macmillan & Co., 1869, pp. 345. 
