Dixon — Transpiration into a Saturated Atmosphere. 635 



It miglit appear that, in ascribing the elevation of the sap to the 

 vital activity of the cells adjoining the upper terminations of the 

 vascular bundles of plants, Tve do aTvay with the necessity of assuming 

 that the water of the transpiration current is in a tensile state. This, 

 however, would be a grave error. It is evident that the effects 

 referred to in this paper, constitute a source of tensile elevation only. 

 All observations, experimental and structural, demonstrate the fact 

 that the water of the transpiiation current is drawn, and not pressed, 

 through the capillaries forming the water conduits of plants. 



The advantage of a periodic pressure in the sap, such as is observed 

 during bleeding, has been pointed out already.^ 



CoBTCtTTSIONS. 



1. The elevation of the water of the transpiration current, when 

 the leaves are surrounded with a saturated atmosphere, is effected by 

 pumping actions proceeding in the living cells of the leaves. 



2. The observations on the drying back of branches furnished with 

 dead leaves renders it highly probable that these vital pumping actions 

 are partially or wholly responsible for the elevation of water even in 

 an unsaturated atmosphere. 



3. These pumping actions are capable of raising the water against 

 an external hydrostatic pressure. 



4. In common with other vital actions, they are accelerated by a 

 moderately high temperature, and are dependent on the supply of 

 oxygen. 



5. The cells adjoining the terminal portions of the water conduits 

 appear to possess this activity, and, in plants provided with water- 

 glands, the pumping actions are not limited to the secreting tissues of 

 these glands. 



^ Ascent of Sap., Phil. Trans. Eoy. Soc, vol. clxxxvi., p. 572. 



