THE CONDUCTION OF WATER. I 



45 



LECTURE V 



THE CONDUCTION OF WATER. I 



Since certain parts of the plant give o§ water and other parts absorb it, 

 conduction of water must of necessity take place in the regions lying between. 

 Absorption, evaporation, and conduction of water may be carried out, under 

 certain circumstances, in different parts of a single cell. On clay soils there 

 is occasionally to be found an alga, Botrydium granulatum, which consists of 

 a spherical green body, about the size of a pin-head, attached to the substratum 

 by a number of colourless branches (Fig. 9). The spherical green region may 

 be compared to the shoot of a higher plant, and the 

 branched colourless region to the root, but the whole 

 structure, being destitute of partition walls, may be 

 looked upon in a sense as a single cell. Similarly 

 the unicellular fungus, Pilobolus, has a root-system 

 distributed throughout the substratum, and a club- 

 shaped aerial portion at the termination of which 

 the fruit is ultimately formed. Let us now sup- 

 pose that transpiration is set up in one of those cells, 

 which we will assume to be saturated with water ; 

 obviously the molecules of water will in the first 

 instance be withdrawn from the membrane covering 

 the aerial region ; the membrane, in other words, loses 

 its water of imbibition. In consequence of this, forces 

 will be generated in the membrane which will induce 

 a withdrawal of the water in the neighbourhood held 

 firmly by the protoplasm. The protoplasm in its turn 

 will seek to make good its loss by absorbing water 

 from the vacuole, thus bringing about a higher con- 

 centration in the contents of the upper parts of the 

 vacuole. Owing to diffusion, however, uniformity in 

 concentration in the contents of both ends of the 

 cell is re-established and the disturbance in equili- 

 brium is transferred to the regions where renewed ab- 

 sorption of water from the soil may take place. 



Let us now consider a somewhat more complicated case by assuming that 

 the Botrydium has been subdivided into two parts by a partition wall separating 

 a green transpiring region from a colourless absorbing region. The first effect 

 of transpiration will, just as above, lead to the concentration of the cell- 

 sap in the green cell. This cell, however, abuts, not directly on water but on 

 the colourless cell ; from it, in obedience to the law of osmosis, the green cell 

 will absorb water, and will continue to do so until the fluids in both cells 

 attain the same degree of concentration. Equilibrium, however, is never 

 reached so long as the green cell continues to transpire and so long as the 

 colourless cell is in contact with water ; for every withdrawal of water from 

 the colourless cell is followed by absorption of water from the soil, and every 

 entrance of water into the green cell is succeeded by renewed transpiration. In 

 this illustration the osmotic attraction arising from the evaporation from one 

 cell is simply transferred to the other, and thus there is really no essential 

 difference between the first example we studied and this one. The case be- 

 comes slightly more complicated, if we take into consideration transpiration 

 as exhibited by a multicellular fungus, such as Penicillium, a plant which is 

 partly rooted in the substratum and partly exposed to the air. 



Fig. g. Botrydium granula- 

 tum. X 25. (After Rostafinski 

 in Sachs's 'Lectures', first ed., 

 fig. 2.) 



