274 THE MOVEMENTS OF WATER 



water is rendered possible, and indeed our knowledge of the cell mechanisms 

 at work is in many cases quite incomplete. 



It is never easy to say whether we are dealing with an active or 

 a passive excretion of water ; moreover a combination of the two is always 

 possible, even in the case of nectaries, in which the plasmolytic excretion 

 of water is most marked, and indeed appears to be sufficient for all 

 requirements. On the other hand, the process of excretion may be 

 primarily an active one, although the excreted fluid may contain large 

 amounts of dissolved substances, and the extracellular dissolved substances 

 must in all cases exercise an effect proportionate to their osmotic energy. 

 A very dilute solution may possess an osmotic energy corresponding to 

 a statical pressure which is by no means inconsiderable (cf. Sect. 46), 

 although it may be insufficient to produce any marked exudation-pressure 

 (as is the case, for example, in Vitis). When no such comparison is possible, 

 or when, as is often the case, the water is driven forth with only relatively 

 feeble energy, special researches are necessary to determine whether the 

 flow of water is produced directly by the vital activity of living cells, or is 

 caused by the presence of a dilute or concentrated solution of osmotic 

 substances outside the cell. It must clearly be understood that an osmotic 

 pressure acts in a precisely similar way to an equivalent mechanical pressure 

 in causing a filtration of water through the cell-walls or tissues, so that there 

 is no essential difference between a flow of water caused by a change in 

 either the mechanical or the osmotic pressures acting upon a cell. 



When a copious excretion of water is stopped by a slight decrease of 

 turgidity, we may conclude with comparative certainty that the pheno- 

 menon was due to an active excretion of water by living cells. This 

 conclusion does not, however, necessarily hold good for those cases in 

 which the water is forced out with only feeble energy, or in which the 

 water slowly oozes through walls which are only permeable with difficulty, 

 for it is then possible that a slight increase in the rate of transpiration 

 may have induced a decrease in the turgidity, and at the same time have 

 caused the dilute extracellular saline solution to dry up, so that it ceases 

 to exert any plasmolytic action in withdrawing water. In such a case 

 the decreased turgidity and the cessation of the plasmolytic action would 

 only be accidentally correlated with one another, so that the stoppage of 

 the flow of water would by no means indicate that the process was one 

 of active exudation. Moreover, as a dilute solution evaporates and becomes 

 more concentrated, its osmotic energy increases enormously, so that the 

 flow of water would increase although the internal conditions remained the 

 same. The external solution rapidly dries up as soon as water in only 

 small quantities can be withdrawn from the contiguous cells, and when 

 this occurs the salts may not be deposited on the surface of the plant, 

 owing to the fact that the cell-wall can imbibe small amounts of saline 



