/^I_|. MOLECULAR FORCES IN THE PLANT. 



above with a lid in two portions, and surrounding the stem so as completely to cover 

 the earth in the pot. If the soil is dry the plant withers. If a bell-glass is placed over 

 it the plant revives, and again withers if it is removed. This shows that the withering 

 is the result of increased, the revival the result of diminished evaporation from the 

 leaves when the roots convey but very little water to the plant. If cut shoots are 

 allowed to wither and are then suspended in air nearly saturated with aqueous vapour, 

 the leaves and younger internodes again revive, although the whole shoot continues 

 to lose weight from evaporation. This phenomenon results from the water passing 

 from the older parts of the stem to the younger withered parts, as must be concluded 

 from Prillieux's experiments ^ 



Sect. 3. — Movements of Gases in Plants^. All growing cells of a plant, or 

 all that are otherwise in a condition of vital activity, are continually absorbing atmo- 

 spheric oxygen and giving back in its place a nearly equal volume of carbon dioxide. 

 The cells which contain chlorophyll have in addition the property, under the influ- 

 ence of sunlight, of absorbing carbon dioxide from without, exhaling at the same 

 time a nearly equal volume of oxygen mixed with nitrogen. In proportion to the 

 activity of the chemical processes which take place within the cells, the movements 

 of gases occasioned by them vary greatly in rapidity. The formation of carbon 

 dioxide at the expense of the atmospheric oxygen takes place continuously and 

 in all the cells; but the quantities concerned are small in proportion to the large 

 amount of carbon dioxide which is decomposed in the green tissues, and in ex- 

 change for which equal volumes of oxygen are exhaled. Some idea of the activity 

 of this last-named process is obtained by reflecting that about one-half the (dry) 

 weight of the plants consists of carbon which has been obtained by the decompo- 

 sition of atmospheric carbon dioxide in tissues containing chlorophyll under the 

 assistance of light. 



Oxygen and nitrogen are permanent gases, as also is carbon dioxide within the 

 limits of the temperature of vegetation, and indeed far below it. Aqueous vapour, 

 on the contrary, is only produced from water within these limits, and under certain 

 conditions even returns to the liquid state. In other respects aqueous vapour be- 

 haves just like oxygen and nitrogen in reference to the processes to be considered 

 here. 



When the gases with which we have to do are traversing closed cell-walls, 

 expanding w^hen diffusing^ themselves through the cell-sap, or permeating or escaping 

 from the protoplasm, chlorophyll-grains, &c., their motion is a molecular one of 

 diffusion. When they fill in their elastic condition the intercellular spaces, vessels, 

 cells destitute of sap, or the large air-cavities among the tissues, it is a movement of 

 the mass depending exclusively on expansive force. The movements of diffusion 

 tend to bring about conditions of equihbrium which depend on the coefficients of 

 absorption of the gas by a particular cell-fluid, on ^the molecular condition of the 

 cell-wall, &c., on temperature, and on the pressure of the air. But these conditions 

 are continually varying ; and the equilibrium which is aimed at is being still more 



' Prillieux, Comptes Rendus, 1870, vol.11, p. 80. 



2 Sachs, Handbuch der Experimental-Physiologie, p. 243. — Miiller, Jahrb. fur wiss. Bot. 

 vol. VII, p. 145. 



