478 



dry and healthy tissue was sharp and, in tlie latter, the vascular bundles not 

 discolored. Hansen's explanation is that "the current of air fast robs the 

 vascular bundles of their water, and so changes them that they can no longer 

 act as conductors. Thus the mesophyll dries at this place." This might 

 also be the secondary process and the drying of the conducting cords the 

 primai7 one, while as yet the drying of the parenchyma of the edge is 

 usually looked upon as the direct eflfect. In op])osition to this, Hansen 

 says: "If one wishes to assume that the wind directly attacks the mesophyll, 

 then it would not be possible to understand why the process of drying should 

 not begin also in the middle of the lamina." 



llruck^ takes up the matter in the same way. He observed that in gen- 

 eral onl}' those leaves with the secondary veins extending to the edge, suf- 

 fered peripheral injury, the so-called caspedodromous or cheilodromous 

 (extending to the edges) venation. (Fig. 96.) Tree leaves from the same 

 region, which did not exhilnt the injury, had "more or less camptodromous, 



or rather brochidodromous, vena- 

 tion ; their course is curved or looped 

 without ending at the edge of the 

 leaf." In the latter form of vena- 

 lion. Bruck percei\ed a decided pro- 

 tection of the leaves against drv'ing 

 from wind. Browning of the vascu- 

 lar bundles is very similar to that 

 produced by frost. 



According to my studies on the 

 production of dry edges of leaves as 

 the result of the action of gases, the 

 process of dying was different here. 

 In the action of the gases in smoke, 

 the tissue did not liecome translucent previously and the walls of the bast 

 elements color yellow to brown ; the cell content dried together as a whole to 

 an approximately uniform substance. The vascular bundles of the peri- 

 pheral zone also were altered, but I explained the earlier death of the 

 peripheral leaf mesophyll by the fact that even if the fine ends of the vascu- 

 lar bundles Still supplied water normally, this was not sufficient to cover the 

 increased loss due to the action of the acid. It might be just the same in 

 the dried edges due to the wind. The evaporation in the mesophyll, increased 

 by the wind, may very well be the primary process. The loss of water in 

 the leaf is relatively greater at the edge, since the upper surface is too large 

 in proportion to the tissue mass and the water conducting system consists of 

 too few elements, i. e., is insufficient. At the places where the leaf is thicker 

 and the venation more developed, the tissues receive more water and retain 

 more, since here the same evaporating surface, as at the leaf edge, has a 



Fif?. 96. 

 Crasprdodiomous Ciimptodromous 

 venation. venation. 



(After Bruck.) 



1 Bruck, W. F., Zur P'rage der Windbeschadlgungen an Blattern. 

 Bot. Centralbl. Vol. XX, Section 2, Sep. 



Beihett 



