530 



It the leaf surface is injured In' frost, lait tlie part of the leaf lying 

 helow the abscission la>er. i. e.. the leaf stump, remains alive, the frozen 

 part of the leaf will dry ui), but the leaf base will be found intact and turgid. 

 Between the leaf base and the dried ])art differences in tension must arise 

 which lead to the loosening of the leaf body. 



Experiments made by l*runet' show how c}uickly the parts injured by 

 frost have dried up. A frozen \ine branch with four leaves placed in water, 

 evaporated 475 mgr. of water within two hours. In this, its loss in weight 

 amounted to 14.46 per cent. Under the same conditions a similar branch, 

 not injured by the cold, evaporated only 132 mgr. of water and, because of 

 the absorption of water which had taken place simultaneously, increased its 

 weight by 0.26 per cent. 



Wiesner has also shown experimental])- how. in jjlants which retain 

 their frozen foliage for some time, often for the winter, this may occasion- 

 ally be based on a rapid drying. He took branches of Ligustnim ovalifoliiiin 

 with frozen leaves and placed them in a warm room in such a way that the 

 sprouts could constantly soak up water. After 6 to 12 days, these dro])ped 

 ihcir leaves while the leaves of shoots not provided with water remained 

 firmly attached. In cases occurring out of doors, where the dead foliage 

 remains in place on the branches, the separation takes place only after the 

 destruction of the tissue. The moldering of the membranes within the 

 abscission layer will gradually advance so that the wind or some other 

 mechanical cause finally brings about the breaking off of the leaf. In these 

 moldering processes micro-organisms will doubtless cooperate. 



From what has been said, it is clear that the mechanics of separation 

 in the autumnal senile defoliation, as well as in that due to frost, can often 

 (liff'er e\en in the same individual, according to the age of the leaves and 

 the existing accessory circumstances. In many plants (grapes), besides 

 the loosening of the whole leaf from the axis, the loosening of the leaf 

 blade from the petiole also occurs. In other disturbances also, this region 

 is especially susceptible and at times manifests its similarity to the base of 

 the petiole through a similar discoloration. For example, in poplars, it can 

 be observed that in tlic autumn the base and tip of the [)etiole become red 

 while the remainder is yellow. 



The diiTerence in the time when these i)rocesses set in in dift"crent indi- 

 viduals, and in the same individual at different heights of the various 

 Itranches. is connected with the physiological age of each leaf. The yonnijer 

 this is. the later it falls from the branch, under otherwise equal conditions, 

 as Dingier'- has determined, by pruning experiments. He observed a 

 greater power of resistance in the young leaves, especially to autumn frosts. 

 The young leaves of Carpinus Betuhis did not freeze during frost periods. 



1 Prunet, A., Sur los modifications de I'absorption et de la transpiration, qui 

 surviennent dans les plantes atteintes par la g-elee. Compt. Rend. d. I'Acad. des 

 Sciences 1892, II, p. 964. 



- Dingier, Hermann, Versuche und Gedanken zum herbstlichen Ijaubfall. Ber. 

 d. D. Bot. Ges. 1905. Part 9, p. 463. 



