ASSIMILATION 
281 
bubbles begin to show, place an inverted funnel over the algae. Fill a 
test tube with water and invert it over the upright stern of the funnel. 
When an inch or more of the gas has collected in the test tube, remove 
it carefully and thrust a glowing splinter into it. Increased brightness 
of the glow or bursting into flame shows that oxygen is present. 
246. Digestion of Food. — Although the plant manufac¬ 
tures food from raw materials, it cannot make use of it until 
it has been digested. The process of digestion takes place 
chiefly in the leaf and the digested food is carried to all 
parts of the plant which need it through the vascular bundles. 
There are many facts about digestion in plants which are 
not so well understood as this same process in animals. It 
is known, however, that the enzyme, diastase , which digests 
starch, is secreted by the protoplasm of the cells and that 
it is very similar to ptyalin, the ferment in the saliva of 
animals which digests starch. 
247. Circulation. — Circulation in plants differs from circu¬ 
lation in animals in not having any central organ for keep¬ 
ing the fluids in motion and in the substances which the 
vessels contain. The xylem of a vascular bundle (Figure 255) 
carries water taken up by the roots from the soil to the leaves, 
where it is combined with carbon from the air and built up 
into foods. The digested foods are carried from the leaf 
to the parts of the plant which need them in the phloem part 
of the vascular bundle, so that, generally speaking, we have 
an ascending stream of water in one part of the bundle and 
a descending stream of digested food in the other part. 
The term sap is used to include both. Sap is moved by the 
combined influence of osmosis (root pressure), transpiration, 
and possibly other factors. 
248. Assimilation. — This is the actual taking up by the 
plant cells of such parts of the digested food as they need. 
It is a building-up process which results in the growth of the 
plant till it reaches the stage of maturity and in maintaining 
it at that size. 
