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 

 foliage-leaves and similar organs, or in the wide air-canals of hollow stems, or in the 

 lacunae 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 plants, 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 diffusion. 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 Equtsetum 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 



