THE VITAL AND PHYSICAL FORCES. 
735 
happen to be subjected ; and the duration of its life being inversely proportional to 
the rapidity with which these operations are performed. 
In addition to the phenomena included under the general term of growth or de- 
velopment, we have to study those which constitute the proper generative process. 
This appears to consist, as the author has elsewhere shown*, in the reunion of the 
contents of two of tlie cells which had been previously separated by the process of 
fission. In the simplest tribes of plants, it would seem as if all the cells thus 
springing from the same primordial source, were capable of performing the genera- 
tive act by “ conjugation;” but this capability, like other endowments, is limited in 
the higher plants to particular groups of cells, which are developed in organs distinct 
from those concerned in the acts of nutrition, and are obviously set apart from the 
first for the performance of the act of generation alone. 
Various kinds of motion, again, are performed by the agency of the vegetable 
structure, although these are less remarkable than they are in animals. From the 
extended researches of Prof. Schleiden it appears probable that within every cell, at 
some stage of its formation, a circulation of fluid takes place-f', which is sustained by 
agencies that cannot be regarded as mechanical, and that are intimately connected with 
the formative processes; varying in its rate with the general activity of those pro- 
cesses, and only ceasing with their cessation. Wherever a cytoblast exists, the cur- 
rents radiate from it and return to it again ; in other cases (as the Chara) they are 
observed to extend over the whole lining of the cell-wall ; in both cases being con- 
nected with the part that shows the greatest vital activity. The zoospores of various 
inferior Algse are covered with cilia, by the vibration of which these bodies are car- 
ried through the water, and deposited at a distance from their parent. These zoo- 
spores are minute cells, formed within certain cells of the parent fabric, and liberated 
by tbeir rupture. So, again, the spiral filaments long since observed in the Chara- 
ceae, the Hepaticse, and in Mosses, and more recently discovered in the Fucacese 
and in Ferns, have a peculiar independent movement, which seems obviously des- 
tined to diffuse them, and thus to bring them into contact with the germ-cells which 
they are to fertilize. Each of these spiral filaments is developed within a distinct 
cell. And lastly, in certain plants, both of high and low organization, sensible move- 
ments are occasionally to be witnessed, which are immediately due to changes of 
form in their component cells ; these changes of form being sometimes spontaneous, 
that is, occurring as a part of the regular series of the vital operations of those cells, 
and not directly excited by any external influences, as we see in the rhythmical 
movements of the Oscillatoriae and being sometimes consequent upon mechanical 
or other irritations applied to the cells which exhibit them, as in the case of the clo- 
sure of the fly-trap of the Dionaea, but not being at all the less dependent upon the 
vital endowments of those cells, which cease to exhibit them when their vital activity 
is diminished. The folding of the leaves of the Mimosa pudica appears to take place 
* Brit, and For. Med.-Chir. Review, Oct. 1849, p 346. 
t Principles of Scientific Botany, p. 95. 
