PLANT NUTRITION. 31 
The first 1esult of this assimilation, chemists tell us, is 
the formation of a soluble substance, “ glucose,” allied 
both to starch and to sugar, and which, or a portion of 
which, becomes starch, and is stored up for future use in 
that form. No starch is formed in an atmosphere pur- 
posely deprived of carbonic acid by the experimenter, 
even if the cell be exposed to the light. Moreover, any 
starch that may have been previously formed disappears 
under such circumstances, just as it would do in dark- 
ness, where the plant is dependent on its reserve stores 
for its nourishment, and not on those which it procures 
directly for itself when exposed to light in an atmosphere 
in which carbonic acid gas forms a part. The changes 
in question are presumed to take place, not in the pro- 
toplasm itself, but in the chlorophyll grains; at any rate, 
it is in them that the starch first makes its appearance. 
It is certain, also, that only cells which contain chlor- 
ophyll—and then only when exposed to light—can 
directly assimilate carbon, Cells without chlorophyll, 
such as those of fungi, obtain their carbon by more indi- 
rect and complex means. The vital importance of the 
exposure of the leaves to sunlight might be inferred from 
the bending of the stems and branches to the light, and 
placing of the mobile leaves at such an angle as to receive 
the full benefit of the sun’s rays—matters which will be 
spoken of further on. 
What the Leaves do in Darkness.—Inhalation of 
Oxygen.—In darkness (as well as under the influence of 
light, in the case of those cells that do not contain chlo- 
rophyll) changes go on of a different character to those 
just described. There is, in fact, a constant elimination 
of carbonic acid gas, and a corresponding absorption and 
retention of oxygen gas. The interchange of these gases 
has been compared to the corresponding changes in the 
case of the respiration of animals ; but doubts have been 
