411 



sequent drying. This view is supported also by the observation of Griiss, 

 that, in Aesculus Hippocastanum, the young leaves, known to be very 

 thickly pubescent, will develop normally after the removal of the bud cover- 

 ing. The effectiveness of the resin covering is seen from an example of 

 Abies Pinsapo, Boiss. When the resin had been removed from the buds 

 by carbon disulphid, the}^ dried up. 



It may now be asked how such irregularities in the unfolding of the 

 buds can be combatted practically. 



The formation of the bud covering cannot be influenced and the danger- 

 ous fluctuations in temperature and atmospheric moisture in spring cannot 

 be controlled. Nevertheless, we think a precautionary measure might in- 

 deed be adopted in forestration in order to moderate the extremes of trans- 

 piration. In the first place, the soil should retain its natural covering of 

 moss or litter, since in this way the soil moisture is preserved, and a damp 

 atmosphere made possible. Hence it might be advisable not to clear away 

 all the leaves, etc. Finally, however, and especially in younger plantations, 

 it might be advantageous to retain protective forests on the side of the tract 

 exposed to the strong spring sun. Among such protective trees the rapid 

 and loosely growing birch is especially useful. 



In garden plants, naturally, one can control conditions very much 

 better. In this conne,ction, attention should be called for the present only 

 to the fact that one should not attempt to replace the uniformly great loss 

 from transpiration by increasing the water at the roots. That does not work 

 well and plants are found to dry up which have an excess of water at the 

 roots. The only natural means is artificial shading. 



Defoliation Due to Heat. 



Observation shows that every year from spring on the foliage falls 

 from our deciduous trees. In city planting this is especially noticable in 

 Acer Negundo and the slightly developed inflorescences of the linden show 

 this almost at once, sometime before the "linden blooms." The process is 

 less striking, but constantly present in other deciduous varieties. Wiesner^ 

 gives this constant dropping of separate yellow leaves the special name of 

 "the summer defoliation" and sees its cause in the changes in the sun's 

 altitude. I think that other causes can also operate here, for, while the 

 summer defoliation usually sets in predominately after the 21st of June, 

 observations show that, for example, according to Wiesner's statements, in 

 Acer Negundo, Acer Calif ornicum. and related species, the leaves first 

 formed may be dropped even in May and at the beginning of June. 



As long as this loss of leaves is slight in comparison with the whole 

 foliage of the tree, it has no pathological significance. Experiments have 

 shown that it is a perfectly normal phenomenon for the leaves on a branch 

 to complete their cycle of growth at different periods. Thus some would 



1 Wiesner, Jul., tjber Laubfall infolge Sinkens des absoluten Lichtgenusses 

 (Sommerlaubfall). Ber. d. D. Bot. Ges. 1904, p. 64. 



