266 H. POLSTER 



also measured by URAS instruments. During the test days evaporation on 

 the open steppe reached maxima of more than i-2 cc. H2O per hour. As 

 the accompanying photographs will show, the crown formations of the 

 two oak species are fundamentally different. As an 'out-post' of the steppe, 

 Qnerciis rohur has formed a globose crown (Fig. 8), whereas Qtiercus cerris 

 has a thin-fohaged, largely extended one (Fig. 9). 



With both species we investigated comparatively the gaseous exchange 

 of the fohage of the southern crown side, of the crown's interior, and of the 

 northern crown side. Consider first the daily course of the thin-fohaged 

 Quercus cerris (Fig. 10). While in the forenoon the southern crown side is 

 most active in both assimilation and transpiration, it is quite the reverse in 

 the afternoon: at highest insolation and evaporation the leaves of the 

 crown interior assimilate and transpire more intensively than those of the 

 southern crown side. The excessive insolation together with the high 

 temperature lead to stomatal closing and rapidly faUing assimilation, as is 

 shown by the transpiration and assimilation curves of the southern crown 

 side. The highest assimilation rate is found in the crown interior in the 

 afternoon, whereas in northern, shaded, side of the crown, photosynthesis 

 remains at a low level during the whole day. On the other hand the thick- 

 foHaged common oak {Quercus rohur. Fig. 11) behaves quite differently in 

 these extreme site conditions. The crown interior of that species is much too 

 shaded for a great activity of gaseous exchange to be possible. But it is not 

 the southern crown side either which shows the strongest assimilation, but 

 the northern crown side, which is protected from insolation. As the colunms 

 show, neither Quercus rohur nor Quercus cerris-in 'outpost position' and 

 under conditions of summer chmate in the Hungarian steppe-reach their 

 highest assimilation rates at the southern crown side, where there is 

 maximum exposure to hght, but always in that part of the crown which 

 possesses the best metabohc activity in moderate shade. Because of the low 

 light required for maximum assimilation, tliis occurs in Quercus rohur on 

 the northern crown side which hardly receives any direct sunlight during 

 the whole day, whereas, with Quercus cerris, it is the interior of the crown 

 which profits in the light shade of its own fohage from intense side insolation 

 in the morning and in the afternoon. 



To return to the five-year-old black poplar stand, we determined the 

 daily amount of assimilation in the base and top regions comparatively for 

 the stand borders and the stand interior, during the drought period and 

 after rain (Table i). The weak efficiency of photosynthesis of the southern 

 border trees and the liigh efficiency of the northern border ones is readily 

 recognised. After rainfall assimilation is generally higher but still the 



