CRITERIA AND METHODS 



123 



From the curve it is fairly clear that the maximum con- 

 ductivity of the sap is found shortly after full bloom, and 

 probably at the time of minimum carbohydrate content of 

 adjacent parenchyma. As Anderssen states, this is a time 

 of high respiratory activity in these tissues. It is likely, 

 therefore, that carbohydrate starvation at this period 

 makes the cells incapable of retaining their solutes which 



6000 

 5600 



5200 

 4800 

 4400 



I 4000 



c 3600 



E 

 V) 



J 3200 



o 



S 2800 

 to 



2400 



2000 



1600 



1200 



JunelOJuly Aug. Sept Oct. Nov. Dec. Jan. Feb. Marcti April May 



i T 



Leaves Shed Full Bloom 



Fig. 8. — The specific resistance of tracheal sap from pear branches. (From 



Anderssen.) 



therefore leach into the xylem. Transpiration can hardly 

 be taking place at a very high rate at this time, so it is pos- 

 sible there is no great amount of transport. In fact, 

 Anderssen states that leaves begin to develop at or shortly 

 after the period of full bloom. Evidence from ringing 

 experiments clearly demonstrates some transport of such 

 solutes in the transpiration stream. This positive evidence 

 of the presence of solutes in the tracheae and transport 





