MUEEAY ON THE COUESE OP THE SAP. 
9 
Now on what ground are we to hold that the reason why the 
plant does not assimilate is the absence of light in preference to 
the absence of food ? Either will account for it, and one will suit 
Sachs’ theory, but the other not. No doubt the food is not taken 
up during the absence of light, but not necessarily because of its 
absence ; it may very well be, although it may be difficult, to show 
it experimentally that if the plant were placed in such conditions 
that it could take food generally, although secluded in darkness, 
we should find that assimilation went on as well in the dark 
as in the light; and this, in fact, is just what Nature does with 
tubers. They are in darkness while the plant is in light, and its 
feeding functions in full operation. They participate in the supply 
of food, although themselves in the dark, and from that source, as 
I view it, they receive and assimilate matter in as great abundance 
as any Apple in the blaze of sunshine. Sachs would say that 
the assimilated matter is drawn from the store produced by the 
plant generally. Eut what is this but to give the thing to be proved 
as part of the proof. He has to prove that assimilation cannot take 
place except in light. I offer an instance of its apparently taking place 
in darkness, and the reply is that that cannot be, because assimila¬ 
tion cannot take place in darkness. It is not that the fire will not 
burn because it is dark, but that it stops burning when the coals 
are consumed, because no more are supplied to it. During the day 
the light and heat of the sun draw up the sap to all the terminal 
parts of the plants, such as the axial extremities of the branches, 
the buds, the leaves, the tubers (which are only subterranean buds), 
where it is partly evaporated and partly assimilated—and as it is 
used up the roots absorb a corresponding flow to supply the con¬ 
sumption ; but at night, when the motive-power is withdrawn, the 
upward flow of sap ceases, the roots become inactive and cease to 
feed, and assimilation ceases. At the same time there is nothing 
to hinder growth going on—it may pile cell upon cell, whether the 
machine is working or not, and it does so. 
Allow me, however, further to cite a well-known fact in favour 
of my views, which it reflects no credit upon us not to have sooner 
so interpreted. Here am I narrating, and you listening to my 
clumsy experiments, and yet we have all had before our eyes—and 
our ancestors for ages have had them before theirs too—a constant 
series of beautiful and conclusive experiments, proving much more 
clearly than I have done what I have attempted to show. I allude 
to what we see in the case of grafts. We know that the stock has 
certain properties differing from those of the scion. We all know 
