The Chicago University Text-Book. 
357 
Barnes that intake of water at the root-surface and intake of 
salts in solution are quite separate processes is that this very 
intake of water must concentrate the solution on the surface of a root 
growing in soil till its salt-concentration possibly exceeds that of 
the root cortex and a diffusion-gradient in the right direction is 
established. Apart from such considerations our knowledge of 
protoplasmic permeability, its variations and mechanism, is now 
advancing rapidly and we see that ignorance is not a safe ground to 
build on. It is quite possible that the “translocation” of salts 
is really some special process not yet understood and as effective 
upwards as the downward translocation of organic substances in 
the phloem, which is carried on without an actual mass-current. 
Barnes admits that transpiration is “ a possible advantage ” in 
one way (p. 326) by causing a relatively rapid mass movement of 
solutions in that part of the ascending water-path which is not 
concerned with protoplasm and osmotic phenomena. Since in a 
tree fifty feet high the path of a water-molecule in the transpiration 
current lies for only a few millimetres among living cells (in root- 
cortex and in tnesophyll) and for ten thousand times that distance 
among dead conduits, it follows that any advantage due to trans¬ 
piration in this major portion of the route must bulk largely in the 
total summing-up. 
It is unexpected to find in the treatment of exudation of water 
no echo of Lepeschkin’s classical paper of 1906 which expounded 
so clearly the mechanism of exudation in the sporangiophore of 
Pilobolus. 
Chapter III deals with nutrition and opens at once with a 
characteristic long discussion of the exact sense in which one should 
use the word “food” in connection with green plants. Is it 
legitimate to speak of carbon-dioxide and salts as the food of green 
plants and say these plants live on inorganic food, or must we say 
that these ingesta are only raw materials and that from them the 
plant makes its own food ? Obviously the full conception of “food,” 
so clearly understood in relation to animals, does not apply to plants, 
and there are difficulties in any use of the word. Yet we think the 
student with a physical training will revolt more against the idea of 
any organism making its own food, as contrary to the “conservation” 
of something or other; and rightly indeed, for from the point of 
view of energy the plant does not make its own food, but the energy 
descends upon it from above, like manna in the wilderness. 
Such a discussion is of course not useless, but the author’s 
