PANPHOTOMETRIC LEAVES 125 



In all such plants the leaf takes up a position which 

 avoids the strongest illumination, but it is quite unlikely 

 that the plant is benefited by receiving less light. It is 

 much more probable that the profile position has its use 

 in preventing overheating and also excessive transpira- 

 tion. An exact investigation for Lactuca Scariola has been 

 carried out by Karsten (19 18). He found that leaves in 

 the horizontal position attained temperatures as much as 

 7'6° C. higher than those standing vertical, both being fully 

 exposed to the sun. A vertical leaf, receiving the sun at 

 right angles to its blade, showed temperatures of 6*3° C. 

 higher than the same leaf when edge on to the sun — in 

 profile position. His results for transpiration are not very 

 easy to interpret, since the data for temperature are not 

 complete. He compares the transpiration for the equal 

 periods from 10 a.m. to 1.30 p.m., that is when the sun 

 strikes the edge of the leaves, and from 1.30 to 5 p.m., 

 when the rays strike more and more at right angles to the 

 blade. The average of the water losses from five plants for a 

 sunny day were, for the first period 5*55 grm., and for the 

 second 6*49 grm. In the first period the air temperature 

 varied from 20*5° to 27° C. ; in the second, from 27° to 22° C. 

 The loss was therefore greater in the second period, but it 

 is not possible to say whether this was due to the different 

 orientation to the sun of the leaves, or to the higher air 

 temperature. With an air temperature of 27° C. about 

 midday, however, it is clear that the leaf exposed at right 

 angles to the sun's rays would take on a very high, and 

 probably dangerous, temperature. 



Wiesner (1907) has applied the term euphotometric to 

 those leaves which are so oriented as to receive the maximum 

 of diffuse light, and panphotometric to those which are placed 

 so that they avoid maximum direct insolation. Reviewing 

 the occurrence of the latter, we see that they form a very 

 characteristic feature in the vegetation of regions of great 

 heat and drought, being specially prominent in the Australian 

 bush. Their occurrence, often in less pronounced form, is 

 frequent in exposed positions in more temperate cHmates. 



