674 
MOLECULAR FORCES IN THE PLANT. 
still solid copper chloride has no time to form a homogeneous solution ; a stratifica- 
tion arises which begins in the lower part of the cell with a very concentrated 
solution, and passes in the upper part into almost pure water when the cell has already 
grown to some height. Since the dilute upper fluid is lighter than the surrounding 
solution, it exerts an upward pressure upon the membrane — just as a cork held down 
under water attempts to rise— till it is ruptured below or at the apex (in the second 
form of cell). But the lighter fluid, when on the point of ascending, becomes at once 
surrounded by a pellicle which remains attached to the walls of the fissure of the old 
one ; and thus apical growth takes place in cells of this description in the form of 
eruptions, just like the formation of branches and excrescences in the round ones. If 
the fluid in the upper part of the cell is pure water, large pieces of the pellicle break off 
and rise up into the surrounding solution like air-balloons open below. If the copper 
chloride is entirely consumed in the formation of the pellicle, the opening caused by 
the tearing oflf of the upper cap does not close, or the whole cell ascends like an air- 
balloon. If rapidly growing cells of the second form are placed in a horizontal posi- 
tion, an outgrowth takes place at the extreme apex as the least solid point, which is 
directed vertically upwards, and then grows in this direction like the former apex of the 
cell. This process, even though it calls to mind distantly the bending upwards of grow- 
ing stems which are placed horizontally (geotropism), bears in fact not the least actual 
resemblance to this phenomenon, as will be shown in Chap. IV; and this is at once 
evident if it is remembered that in these cells there is no such thing as growth by 
intussusception. 
Sect. 2. Movement of Water in Plants \ The growth of the cells of plants 
is always connected with the absorption of water, not only as regards the in- 
crease of size of the vacuole, but the growth of the cell-wall and of other organ- 
ised structures is also accompanied by the intercalation of particles of water be- 
tween the solid micellae. Water must therefore be conducted to the growing cells 
and tissues ; and when the organs which absorb the water lie at a distance from 
those which require it for their growth, the movement which results is necessarily 
considerable. Water is in the same manner required by the organs of assimilation, 
since it furnishes the hydrogen required for organic compounds. The reservoirs 
of food-material in which the assimilated compounds are for a time accumulated 
also require water for the purpose of again dissolving these substances, in order that 
they may be carried as formative materials to the leaves and the growing apices of 
roots and stems. All these movements of water, which are necessarily connected 
with nutrition and growth, proceed slowly Hke growth itself ; their direction is in 
general determined by the relative positions of the organs which absorb the water 
from without and of those which make use of it. 
In plants which grow under water or beneath the ground where no loss of 
water takes place or only to a very considerable extent, there is no need for 
these processes. The case is nearly the same also with some land-plants which 
are almost completely protected by a peculiar organisation from loss of water by 
evaporation (transpiration) when it has once been absorbed, as the Cactus-like 
Euphorbias, Stapehas, &c., which are by this means enabled to live in the most 
arid localities. But the great majority of plants have foliage with a very large 
^ See Sachs, Handbuch der Experimental-Physiologie, the section on the movement of water, 
p. 196, where the literature up to 1865 is mentioned; the most important of the more recent 
publications are quoted in the sequel. 
