692 
MOLECULAR FORCES IN THE PLANT. 
of substances in the plant, assimilation, and growth ; so that a state of rest can very 
seldom occur. The ordinary condition of the gases which are diffused through the 
cells of plants is that of movement. 
But even the masses of gas found in the cavities of plants are not generally at 
rest. By the setting free or absorption of carbon dioxide or oxygen in the cells, the 
equilibrium is disturbed also in the neighbouring cavities ; and changes in the 
pressure of the air or in temperature also exert an influence. The flexions again 
of the stem and leaf-stalk produced by the wind cause compressions and dilatations of 
the gases which fill the cavities, and these again give rise to currents of gas in the 
interior. The rapidity of the movement in the cavities varies greatly in proportion 
to their size ; within the very narrow intercellular spaces of ordinary parenchyma the 
motion is slow and inconsiderable even under considerable pressure, as contrasted 
with the rapid currents which are possible in the large intercellular spaces of most 
foHage-leaves and similar organs, or in the wide air-canals of hollow stems, or in the 
lacunoe of the tissue of water-plants. 
In attempting to collect the most common phenomena into a more definite arrange- 
ment from this general point of view, the following appear to be the more important 
points. 
(a) Unicellular p/ants, as well as those which consist merely of filaments or plates of 
cells such as occur in Algae, Fungi, and Mosses, are in immediate contact with the air or 
with the surrounding water which contains gas in solution. The only essential condition 
here is that the gases shall be able to enter and escape from the cells by diff'usion. If, 
for example, a cell of this kind containing chlorophyll is placed in sunlight, the carbon 
dioxide absorbed by it is decomposed ; a fresh supply of the gas is therefore continually 
penetrating into it from without, because it is prevented from saturating the cell-sap ; 
oxygen, on the contrary, is being constantly disengaged, the cell-sap receives more than 
it can contain, and gives off the excess by outward diffusion. Under these conditions 
therefore two molecular currents are set up in opposite directions which permeate the 
cell-wall, the protoplasm, and the cell-sap ; and since carbonised products are formed in 
the cell at the expense of the decomposed carbon dioxide, this decomposition is the 
simultaneous cause of fresh quantities of the gas perpetually diffusing into the cell. The 
quicker the decomposition of the carbon dioxide, the more quickly is it replaced. The 
conditions are similar in cells containing chlorophyll when in darkness and in cells 
destitute of chlorophyll, though the chemical process is different; they absorb oxygen 
and produce carbon dioxide ; only the process is much slower and less active. The 
cell acts as a centre of attraction for the gas which is decomposed in it, and as a 
centre of repulsion for the gas which is produced in it. This rule holds good also for 
the individual cells of a tissue, only that in this case the processes are more complicated, 
inasmuch as the diffusion currents of the gases do not take place between the cells and 
an unlimited external volume of gas, but between cells and cells on the one hand, 
between cells and internal air-cavities of limited size on the other hand. 
(b) Among plants consisting of complicated aggregates of cells, submerged Water- 
plants are of peculiar interest, because their intercellular spaces do not open outwardly 
through numerous stomata, but communicate with large cavities which are formed in the 
interior of the tissues by the disjunction of cells or by the rupture of their walls. The 
underground stems of Equisetum and of many bog-plants show a similar structure. 
Uninjured plants of this kind are closed and air-tight outwardly ; the gases which 
collect in the cavities can originate only from the surrounding tissues, which absorb 
oxygen, nitrogen, and carbon dioxide by diffusion from the surrounding water. These 
gases cannot simply diffuse through the surrounding tissues, but they undergo change 
within them, and when once collected in the spaces they are still further influenced by 
