LIFE OF THE PLANT-CELL. 89 



36. When free gases are present in the cell in larger quantities 

 than can be held in solution in the fluid, they naturally pass through 

 the cell-wall, which presents no hindrance to their escape. When 

 the fluid is saturated with gas, the nature of the gases in the neigh- 

 bourhood of the cell determines whether, according to the law of 

 equilibrium of gases, a partial interchange takes place or not. The 

 gases which are combined in this way are principally oxygen, car- 

 bonic acid, and hydrogen. 



The most universally present processes in the cells of plants are the 

 decomposition of water, with the fixation of hydrogen, and the decom- 

 position of the assimilated matters by the formation of carbonic acid.* 

 Sometimes, as in the Fungi, the decomposition of water is attended with 

 the liberation of hydrogen, f From hence it arises that, with the water of 

 the plant-cell, the gases which are dissolved in it are also taken up. Thus 

 we constantly find free gases which do not unite as in other chemical 

 combinations, but which must also pass out free. This process occurs in 

 its simplest form in the vegetating cells of the Conferva, where carbonic 

 acid gas is taken up, and oxygen gas is given out as the consequence. $ 

 In this case, the Daltonian law of the interchange of gases cannot be 

 taken into consideration, because the quantities do not correspond with 

 the law. 



A fluid consisting of a solution of equal parts of gum and sugar, when 

 saturated, would contain about 70 per cent, of carbonic acid. When this 

 becomes fixed, about sixty-three volumes must be set free in the form of 

 oxygen gas, so that the carbonic acid is diminished nine-tenths of its bulk 

 by the loss of oxygen. De Saussure's experiments prove that this is 

 about the relation which the carbonic acid and oxygen bear to each other. 

 There are, however, many circumstances which may modify this process 

 to some extent, and especially the interchange of gases according to the 

 law of Dalton. This process is sometimes called, with great impropriety, 

 respiration, and is supposed to resemble the same process in animals. The 

 phenomena become much more complicated when, in addition to the 

 simple process of decomposition which goes on in the cell, some of the 

 contained substances, as, for instance, resin and the like, absorb gases, as 

 oxygen, from without, and unite with them. 



IV. Disposition of the assimilated Matters. 



37. The plant-membrane grows through the assimilated matters 

 in such a way that it is extended equally on every side, so that a 

 still larger space is surrounded, and its walls become thickened. 



The cause of growth in this case is apparently from the attraction of 

 similar substances for each other, as seen in the increase in size of crystals 

 when placed in solutions of the same salt. The absorbed matters do not, 

 however, arrange themselves in regular layers upon the surface of the 

 membrane, but permeate all parts of the absorbent membrane in a semi- 

 fluid state ; but still the increase of surface is greater than of thickness. 

 In this way a cell continues to grow without its walls becoming thicker. 

 We have no grounds to suppose that isolated cells grow through apposi- 



* See Germination. 



t Humboldt, Floras Fribergensis Specimen, p. 179. 



I First observed by Priestley, in the year 1773. See Priestley, Observations and 

 Experiments on various Kinds of Air. 



