674 MOLECULAR FORCES IN THE PLANT. 



still solid copper chloride has no titne 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 oiT 

 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 cfF 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 pecuHar 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-Physiologic, 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. 



