-ycS MOLECULAR FORCES IN THE PLANT. 



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 off 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 earlier 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, and not only as regards the 

 increase of size of the cell-cavity ; 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 molecules. 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 like 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 inconsiderable 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 peculiar organisation from loss of water by 

 evaporation when it has once been absorbed, as the Cactus-like Euphorbias, Stapelias, 

 &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 superficial development ; 

 when the leaves are also delicate, as in most plants with a rapid growth, a very 

 considerable portion of the water of their cell-sap is removed by evaporation 

 within a short time, so that in the course of a single period of vegetation the 

 quantity of water which has been withdrawn by evaporation may exceed many 

 times the weight and volume of the plant itself. It is easy to understand that this 

 is possible only when the loss is compensated by the absorption of corresponding 

 quantities of water through the roots, and that the water withdrawn from the 

 leaves is replaced in this way. As long as the tissue of plants in which transpiration 



' See Sachs, Handbuch der Experimental-Physiologie, the section on the movement of water, 

 p. 196, where the literature up to 1865 is mentioned; the most important of the more recent publi- 

 cations are quoted in the sequel. 



