MURRAY ON T3E COURSE OE THE SAP. 
11 
of the sap, and everything has by degrees been arranged to fit neatly 
into it. That rotten beam was removed and its place supplied by 
Sachs’ theory; that, I think, I have shown to be rotten too, and in 
removing it, without having any other prop to put in its place, 
down must come the hypothesis that the plant derives all its carbon 
from carbonic acid in the atmosphere, or its nitrogen from free 
uncombined nitrogen through the leaves, and all power of taking 
anything into the system through the leaves, and, of course, all 
hypothesis of feeding, whether vegetarian or carnivorous, through 
these organs must be given up too. The theories of circulation by 
imbibition, diastasis, endosmose, and exosmose, for me are already 
defunct. Six weeks unavailing effort to get the slightest indication 
of any of these phenomena in the living plant seem enough for me. 
The current is steadily upward, and not only permits nothing to 
come down against it, but is too powerful to permit anything to 
deviate from its own place, and force its way into another, even 
by uniting with it on the way upwards. My position, therefore, 
is that for a plant to absorb carbonic acid through the leaves for 
the purpose of supplying it with that important element involves a 
physical impossibility, and yet this is one of the best received 
vegeto-physiological hypotheses. It has the advantage of giving a 
glimmer of an explanation how plants may have first originated. 
They consist of carbon, nitrogen, and mineral ingredients, besides 
oxygen, which may be derived from the latter, and it is open to 
say that plants derive their mineral constituents from the degrada¬ 
tion of rocks, and their carbon and nitrogen from the atmosphere— 
and I do not say that some plants (as lichens) do not. But whatever 
laboratory experiments may say, if we test the hypothesis by the rules 
of common sense and common experience, they tell us that you may 
try to grow a plant in mineral ingredients and leave it to get its carbon 
from the atmosphere as much as you like but it won’t grow. As far as 
common people can see it will only thrive in humus, in other words, 
where its roots can draw carbon from the organic matters already 
elaborated in the soil by the long-continued accumulation of past 
ages. But Sachs states the point very broadly. “ The fact is 
unquestionable,” says he, “ partly established by direct researches 
on vegetation, partly inferred from the circumstances under which 
many plants live in a natural condition, that most plants which 
contain chlorophyll (e.g. f our cereal crops, Beans, Tobacco, 
Sunflower, many saxicolous lichens), Algae, and other water plants 
obtain ” (through the leaves—he does not say so here—but it is 
implied, and is of the essence of his theory) “the entire quantity 
