282 TRANSPIRATION. 



produced by transpiration could only reach down to the highest level attained by 

 the water which has been forced up by root-pressure, and precautions must be 

 taken that, in case of the abatement of the root-pressure, water would be raised 

 from the lower positions up to the transpiring cells, and that under certain condi- 

 tions the action of transpiration should reach even to the absorbent cells at the 

 root-tips. It has been shown by experiments that plants with large leaves lose in 

 the summer more water by transpiration than is forced up into the stem by root- 

 pressure, and yet the leaves do not become faded. The conclusion drawn from this 

 is that at certain times the effect of transpiration makes itself felt down from the 

 leaves through the stem as far as the root-tips. It has also been shown that in 

 many plants, just when the most active evaporation is taking place in the leaves, 

 none, or only very little sap is forced into the stem by root-pressure. If the stem 

 of a vine be cut across in the height of summer, when the green leaves have been 

 unfolded some time and are transpiring actively, no " tears " are seen on the cut 

 surface of the stump, no drops are pressed out. The vessels contain rarefied air 

 but no sap, and water can be sucked through the stump by the vessels even in the 

 direction of the root. 



Let us pause here in order to get a clear idea of the relations between transpira- 

 tion and root-pressure. Given the conditions for an abundant evaporation from 

 the aerial portions of a plant — i.e. a fairly dry air, water, and an appropriate 

 development of transpiring surface — then the action of root-pressure is diminished, 

 while that of transpiration is increased, and governs the whole of the movement of 

 the sap. If, on the other hand, the conditions for evaporation from the aerial por- 

 tions of the plant are unfavourable — if the air is very damp, or if the branches of 

 the plant are not yet in leaf^then root-pressure comes into play, and, supported by 

 cells with absorbent contents which occur in the higher regions of the plant, can 

 force up the sap to the tops of trees and to the highest shoots of vine-branches 

 which remain leafless all the winter. So far, therefore, root-pressure can supersede 

 and replace transpiration, a fact of great importance in places where the air is 

 sometimes very damp, and in countries where the trees and lianes shed their leaves 

 in autumn; at the commencement of the next period of vegetation they have not 

 yet put out their new foliage, and therefore do not possess a sufBciently large tran- 

 spiring surface. It is very probable that in the autumn, when preparing for the 

 winter, certain cells in trees and lianes provide themselves with materials by means 

 of which in the coming spring they may exercise a very strong sucking action. 

 This would also partly explain how it comes about that in the spring there is such 

 a strong upward current of sap in the still leafless trees and vine branches, and 

 that the water is conducted up even to the topmost shoots of lianes 100 metres 

 long, which have shed all their leaves in the previous autumn. 



A perfect substitute for transpiration in the form of the pressure produced by 

 the absorbent cells is seen in moulds, in the already-mentioned dry-rot fungus, and 

 generally in leafless cryptogams: possibly also in those orchids possessing neither 

 green leaves nor stomata, and in other humus plants (saprophytes) such as the 



