390 Scientific Proceedings, Royal Dublin Society. 



transmit the sap in tbem, with all its constituents, more rapidly than will the 

 tracheids, when both are under the tension generated by the leaves. But 

 when a bubble arises the stream will be deflected into the tracheidal columns. 

 There, owing to the smaller size of the compartments composing the channels, 

 the tensile column will acquire greater stability. Hence, when the wood is 

 rich in water the most rapid transit upwards will be effected in the vessels, 

 which, at the same time, will be comparatively rich in carbohydrates, while 

 a slower and more dilute stream will pass up in the tracheids. If drought, 

 by causing an. increase in tension, produces bubbles in the conducting 

 channels, the major part of the stream, with its solutes, will have to pass 

 up through those tracheids in which no ruptures (bubbles) have developed, 

 thus securing a stable, if slow, suppljr to the leaves. According to 

 Strasburger's observations (llj during transpiration the majority of the 

 vessels of tlie spring wood in the condiicliug zone are without bubbles. 



The recognition of the glandular function of the wood parenchyma, the 

 translocatory activity of the medullary rays, and the transmission in tlie tracheae 

 of the circulating carbohydrates, affords a satisfactory explanation of tlie 

 presence of living elements among the otherwise lifeless tissue of the wood. 



Summary. 



1. Sugars (monosaccharides and disaccliarides, or both) are found at all 

 times in the sap in the tracheae of the trees examined, and usually in greater 

 quantities than electrolytes. 



2. The greatest concentration of the sugars occurs in tlie early spring ; 

 this is followed by a rapid dilution in spring and early summer, so that a 

 minimum occurs in the summer or autumn. A rise in concentration, slow at 

 first, then takes place through the winter, culminating in the vernal maximum. 



3. The vernal maximum coincides witli the period of greatest root- 

 pressure, and is simultaneous with or just prior to the opening of the 

 leaf-buds. 



4. The rise in transpiration, initiated by the expanding leaves and 

 facilitated by the opening of the conducting channels by root-pressure, is 

 largely responsible for the dilution of the carbohydrates. The falling off 

 and cessation of the transpiration stream in the autumn allow the concen- 

 tration again to rise. 



5. The conveyance upwards of carbohydrates, of which sucrose appears to 

 be the most important, is a continual and primary function of the tracheae. 



