II DEVELOPMENT AND ENVIRONMENT 47 



steadily, but the shape of the curve is only known 

 approximately as yet. 



The rate of mean maximum transpiration obtained in 

 these trials was "00076 g. per sq. cm. per minute, which 

 seems to be within the order of probability. A trial with 

 an entire plant in the same way indicated a rate of 0'0003 

 at midday on June 15th, many of the leaves being shaded 

 by others ; there were twelve leaves, with a total area of 

 585 sq. cm., the wet and dry bulbs reading 24° C. and 

 33° C. respectively. 



Tissue temperatures. — A number of records of the 

 internal temperatures of various tissues have been taken 

 by inserting a thin copper-constant in thermo-electric 

 couple in the tissues ; * it is then balanced upon a 

 similar couple bedded in a rod of paraffin wax, which 

 is kept at shade temperature behind a screen close 

 to the plant. The actual shade temperature is simul- 

 taneously recorded by means of a thermograph near the 

 control couple. 



Since the shade temperatures form the standard of com- 

 parison between various sites, this method is preferable to 

 direct determinations of absolute tissue temperature. ■ The 

 difference between the two couples is automatically 

 recorded by a " Thread Galvanometer," a zero line being 

 first traced in order to show when the couples are at the 

 same temperature. 



The results obtained in this way present a few features 

 of exceptional interest. Variations of it 1 '0° C. are shown 

 even by large bolls, while the stem within five millimetres 

 of the terminal bud varies in the same way as the 

 younger leaves, though with less amplitude. Young 

 leaves, which have not attained to a third of their ultimate 

 length, rarely exceed the shade temperature, but frequently 

 fall below it, their extreme variation being about + 0° to 



* Blackman & Matthaei. 



