SUMMARY. 63 



GENERAL SUMMARY. 



(1) The relative transpiration rates of the plants used differ according to 

 the previous environmental history of the plant; so that conclusions regard- 

 ing the actual transpiration rates of plants in situ can not be drawn from the 

 measurement of water losses from potted plants. Different branches of a 

 single tree differ in relative transpiration rate no more than do the relative 

 transpiration rates of the same branch on different days, and, furthermore, 

 relative rates of different branches agree as well as do the rates of different 

 potted plants taken on the same day or the rates of the same potted plant 

 on different days. Therefore the transpiration behavior of an adult tree 

 may be known better from the measurement of small branches than from 

 the measurement of potted plants. 



(2) The maxima for relative transpiration of potted plants of Parkinsonia 

 microphylla were found to vary directly with the soil moisture, and slight 

 evidence was obtained for the same variation in the case of plants in situ. 



(3) The maxima of relative transpiration varied with the structure of the 

 tissues in the order to be expected from their anatomical structures. 



(4) The actual transpiration of adult trees and of young seedlings growing 

 in the open shows a maximum, which occurs 2 to 3 hours earlier than the 

 maximum of evaporation for the day. This maximum is followed by an 

 abrupt drop and a subsequent rise, the rise being much more pronounced 

 in the case of branches in leaf. This rise is in general great enough to appear 

 as a rise in the relative transpiration. In the case of green-house-grown 

 potted plants of the same species the early maximum appears in the relative 

 transpiration, but not always in the actual transpiration, and a drop appears 

 in various forms all the way from a distinct drop in the actual transpiration 

 to only a slight flattening in the slope of the relative transpiration curve. 

 The early maximum does not appear even in the relative rate when readings 

 are taken in the shade. 



(5) The curve of stomatal behavior follows the relative transpiration 

 curve in such a manner that the existence of an interrelation is evident. 

 When potted plants having a soil moisture of 14 per cent and 3 per cent are 

 compared in respect to stomatal behavior and transpiration, a relation 

 between the amounts of transpiration and the size of stomatal openings is 

 seen to exist, but the amounts are not even approximately proportional to 

 the linear dimensions of the stomatal openings. No conclusions regarding 

 the action of cause and effect can be drawn from the measurements taken. 



(6) Water content determinations of leafless twigs and of branches a 

 meter distant from the twigs show an inverse relation, while the curve for the 

 twigs follows the relative transpiration curve. Curves from further deter- 

 minations of water content of twigs and their leaves show a relation to trans- 

 piration and relative transpiration curves, from which is offered the theory 

 that the drop in relative transpiration and in actual transpiration is due to 

 a slight drying out of the tissues. This theory is further strengthened by 



