372 
POPULAR SCIENCE REVIEW. 
with a denser fluid, and the upward 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 he a residuum of energy still un- 
accounted for connected with the vitality of the plant itself. 
In particular, the selective power of plants in absorbing from 
the soil a larger portion of those ingredients which are re- 
quired 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, i.e., of the solid ingredients derived from the soil, 
will be remarkably different ; while- 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 “ Origin of 
Species,” how unavoidable it seems, in describing the pheno- 
mena of nature, to use language involving the idea of contriv- 
ance 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 in other terms as an expla- 
nation of that law, occurs in an admirable treatise on the growth 
of plants — Johnson’s 66 How Crops Grow.” * “ The cereals are 
able to dispose of silica by giving it a place in the cuticular 
cells ; the leguminous crops, on the other hand, cannot remove 
it from their juices ; the latter remain saturated, and thus 
further diffusion of silica from without becomes impossible, 
except as room is made by a new growth. It is in this way 
that we have a rational and adequate explanation of the 
selective power of the plant.” The “rational and adequate 
explanation ” seems to me, on the contrary, to be merely a re- 
statement of this selective power of the tissues in other terms. 
Because the tissues want the silica, is no explanation of how 
they get it. 
The curious and interesting movements of climbing plants 
have been investigated by Palm, Mohl, and Asa Gray, and form 
the subject of one of the most charming of Mr. Darwin’s works. 
It is well known that climbing plants, such as the hop, honey- 
* “ How Crops Grow : ” A Treatise on the Chemical Compositions, Struc- 
ture and life, of the Plant, for Agricultural Students. By S. W. Johnson. 
"Revised and adapted for English use by A. II. Church and W. T. T. Dyer. 
London : Macmillan & Co., 1869, pp. 845. 
