412 



fall earlier, some later. Those produced first in the spring are weak in 

 their formation, being smaller and not so brightly colored; hence they soon 

 reach their full development, when their assimilation is arrested, as the 

 stronger leaves, produced later, cut off their light. Then the tree frees itself 

 of the organs incapable of working. 



However, the summer defoliation is to be considered as a phenomenon 

 of disease when it becomes extensive and suddenly attacks the well develop- 

 ed foliage in full sunhght. Late frosts and more often a continued period 

 of drought, combined with great heat, are among the causes of summer 

 defoliation. Wiesner distinguishes the latter form as "dcfolialion due to 

 heat," clearly a result primarily of excessive transpiration with an unequal 

 decrease in the supply of v^^ater in the trunk. 



I found examples of defoliation due to heat in the trees planted along 

 the streets, especially among the lindens, in spite of abundant watering. 

 From this it is evident that actually the dry air with abundant sunshine 

 should be assumed to be the injurious factor. With deficient water supply 

 in the soil alone the foliage dies from summer blight but usually remains 

 hanging on the tree. 



The linden, despite its beauty, is not to be recommended as a street 

 tree because of its especial sensitiveness. The summer linden shows earlier 

 and more severe effects than the winter linden, and after the appearance of 

 summer heat, almost without exception, is found covered with the fine webs 

 of the weaving mite (Tetranychiis telarius). In many trees aphids occur 

 in immense quantities. After defoliation, from which only the tips of the 

 branches are excepted, there is manifest a prematurely dormant period. As 

 soon as the weather becomes cooler (or when the streets are abundantly 

 watered during the hot period) a second growth appears in which the de- 

 velopment of lateral buds can push off the hanging leaves {defoliation due 

 to growth, according to Wiesner). In wet autumns the wood of this second 

 growth does not ripen properly and is easily injured by the winter frosts. 



In order to avoid these conditions it is advisable to plant elms rather 

 than Hndens along the streets. If these conditions appear along avenues of 

 older trees, which cannot be replanted, the streets must be sprinkled as 

 frequently as possible. Spraying under heavy pressure in the late evening 

 may prove to be especially useful. I consider that consistently following 

 this measure will prove the most effective prevention of vermin attacks. 



Honey Dew. 



According to observations made up to the present, a disease must be in- 

 cluded here which has often^ been described under the name "honey deii'" 

 {Melligo, Melaeris, Ros mellis) and which has been traced to very different 

 causes. This disease is characterized by the appearance of a sugary coating 



1 Saccharogenesis diabetica; Ungrer, "Exanth. p. 3. — Honning Dugen, Fabricius 

 Kiobenh. 1774. — Le Givre, Adans, cit. bei Seetzen: Sistematarum geneialiorum de 

 morbis plantarum. Gottingae 1789. 



