GROWTH. 45 
chlorophyll in its tissues (and it is with these alone that 
we are here concerned) gains carbon in the form of car- 
bonic acid gas when it is exposed to light, and loses it 
constantly, whether in light or darkness. Nevertheless, 
as the total gain is greater than the loss, the balance is in 
favor of the plant. The plant may thus be considered 
to be the result of the greater amount of work done 
(under the influence of sunlight) through the medium 
of the green cells than of that accomplished by the color- 
less cells, even though their action is continuous, and 
that of the green cells intermittent. It is for us now to 
enquire how this increase of substance, how this growth 
and building up of new materials takes place. The cir- 
cumstances that are propitious, or which are antagonis- 
tic to it, have been considered ; it remains to enquire 
into the way in which the process itself is effected, and, 
for this purpose, we must revert to the fabric of the 
plant, and go back to the cell. 
Growth of Cells.—The perfect cell, ag has been 
explained, consists of a membranous bag enclosing the 
protoplasm, some liquid contents, and in the growing 
state a small, highly refracting oval body known as the 
“nucleus.” The growth of a cell may take place in one 
of three different ways. There may be simple extension 
of the cell membrane, which becomes stretched by the 
influx of fluid into its cavity, producing a state of ‘‘tur- 
gescence” in the cell. But this turgescence, which is of 
intermittent occurrence, is of itself hardly to be truly 
considered as growth in the sense of the formation of new 
material, although so closely associated with it that no 
true growth can take place without it. 
A second mode of growth, resulting in the real addition 
of material, and consequent increase of weight, is that 
called ‘‘intercalary,” because new material is supposed 
to be intercalated or squeezed in between the old. Thus, 
