2io THE MOVEMENTS OF WATER 



PART I. 



SECTION 34. The Conveyance of Water in Transpiring Plants. 



In a simple mould fungus transference of water from cell to cell 

 suffices for all requirements, whereas in more highly developed terrestrial 

 plants a special conducting system is necessary in order that water may 

 be conveyed in sufficient amount and with sufficient rapidity to replace 

 the loss by transpiration, which is frequently very pronounced (Sect. 33). 

 Water travels very slowly from place to place so long as it depends upon 

 transference from cell to cell in a turgid tissue. Thus cylinders of living 

 pith or strips of cortex having their lower ends dipping in water begin to 

 wither at a height of from 5 to 15 centimetres, even when transpiration 

 is only moderately active l . 



This special transference of water is entrusted to the xylem of the 

 fibrovascular strands, whose branches convey water along good conducting 

 channels to every part, so that only a narrow stretch of the less active 

 intermediate connective tissue need be traversed to replace the water given 

 off by a transpiring epidermal cell of a leaf or stem. The final transference is 

 therefore similar to that in Pemcillium, in which the water passes from cell 

 to cell in order to travel from the absorbing mycelium to the transpiring 

 conidium. 



In both cases it is the difference of potential due to the loss of water 

 which induces the conveyance of fresh supplies to the transpiring parts. 

 When a mycelium absorbs water from the substratum, equilibrium is 

 restored by a flow of water towards the point from which it has been 

 removed, and a similar stream follows any localized removal of water, so 

 that in the fibrovascular conducting system an upward current is main- 

 tained towards the transpiring leaves ; this occurs also even when the 

 water is withdrawn from an isolated fibrovascular strand without the help 

 of the surrounding parenchyma. 



It is not yet clear how this active raising and transference takes place 

 in the conducting tissue. The parenchyma abutting upon the conducting 

 strands withdraws water from them in a similar manner to that in which 

 the roots absorb water from the soil, and in both cases currents are induced 

 towards the absorbing organ by a backwardly transmitted sucking- force. 

 The latter, whether special conducting channels are present or not, reaches 

 to the root and to the soil outside, from which it withdraws water. Hence, 

 as transpiration continues, water is sucked in through the cut surface of 



Thus: Westermaier, Ber. d. Bot. Ges., 1883, p. 371; Sitzungsb. d. Berl. Akad., 1884, 

 Bd. XLVIII, p. i no; Jahrb. f. wiss. Bot., 1884, Bd. xv, p. 627. On Laminaria see Sachs, Arb. d. 

 Bot. Inst. in Wiirzburg, 1879, ^d. n, p. 315. 



