1892. 



THE FALLING OF LEAVES. 687 



and are hence more susceptible to frost. Examination of the leaf-scar 

 often showed torn fragments of tissue attached thereto, evidence that 

 the separation had been a violent one. 



In a paper on the parts involved in the process of defoliation, 

 McNab (16) argues that it is not a separation of the leaf from the axis 

 bearing it, but of one part of the leaf from another, the " epiphyll '' 

 from the " hypophyll." 



His view is based on a theory of Eichler's, that the leaf consists 

 from its earliest appearance of an upper and a lower portion ; from the 

 upper, epiphyll, the stalk and blade are developed, while the stipules 

 are appendages of the lower, hypophyll, which is a part of, and adherent 

 to, the axis and ultimately forms the leaf-scar. This would explain 

 why the stipules sometimes remain after the fall of the leaf, as in 

 Mimosa ; usually, however, they fall sometime before, having served their 

 purpose as protective organs to the developing leaf. This is a 

 question of morphological interest, but the results hitherto obtained 

 show rather that the separating layer is a new formation appearing 

 somewhere, but at no very definite place, in the lower part of the 

 leaf-stalk, and not marking the line of some previous morphological 

 division. 



While the production of the separating layer is the first cause 

 of the leaf-fall, external conditions have also great influence on the 

 process. In still weather a very slender attachment may suffice to 

 support the leaves, which the force of a heavy shower or a gust of 

 wind would speedily send to the ground. In a large town like London, 

 where the air is vitiated by smoke, the leaves will fall sooner than in 

 the pure open country. The glorious autumn tints are also absent, or 

 but feebly reproduced in town, the yellow, orange, or crimson colour- 

 ing giving place to a dry rusty brown. In a Midland pottery town 

 where manufacturers run riot and sneeze at anti-smoke legislation, 

 the leaves fare worse still, and half-way through September don a 

 jaundiced hue and drop sadly to the ground, or shrivel miserably on 

 the branches. 



Climate has naturally a great influence on the length of time 

 during which the leaves will remain attached. Where the autumn is 

 prolonged the foliage will also last. Thus for the neighbourhood of 

 Naples, excepting years when the winter sets in unusually early, 

 Tenore quotes the end of November for the defoliation of the walnut, 

 ash, lime, oak, and poplar, and states that the leaves of the apple, 

 beech, elm, and birch often last till the end of December. Foliage 

 generally will keep longer in a damp, sheltered spot than in dry, 

 sunny, exposed positions, and longer after a wet than a dry summer. 

 All the trees do not lose their leaves at the same time ; the oaks and elms 

 are still green when the chestnuts and limes are almost bare. x\part 

 from conditions of climate or weather, each species has its own 

 peculiarity in this regard". This may sometimes be due to the fact 

 that species now^ growing together in one country have come 



