HOW AND WHY DO LEAX'ES FALL? 



B^ 



S. BOULGER. F.L.S.. F.G.S. 





There is no element which contributes more to the 

 difference of the landscape as we travel from the 

 Equator northward than the prevalent character of 

 the foliage of the trees. In the tropical jungle the 

 bulk of the trees are dark green, thick-leaved e\er- 

 greens, a characteristic which extends northward in 

 the more insular moist climates of coast and island 

 regions, notably exemplified b\- the flora of japan. 

 In the Cooler Temperate 



Zone the predominant trees V; y. '-, ' 



are dicotyledonous angio- \ ■\\VV' : \ 



sperms, the "broad-leaved \ l,',il ■ '.iv.- ' 



trees" of our foresters, with 4\-'. i '^' 'V 



smaller, thinner leaves than I;''"*' '■ ' { V 



those of the jungle, lighter .JL i.i, ^1 i 



in tint, producing a less ^V.i' V'- • 



dense shade, and. for the 

 most part, falling in autumn. 

 Northward of these again 

 the polar limit of arborescent 

 vegetation is reached by the 

 striking Sub-Arctic Zone of 

 " needle-lea\'ed "' conifers, 

 mostK" evergreen. 



It must be noted, in 

 passing, that this term "ever- 

 green." though often true 

 enough of a tree, does not 

 apply to the individual leaf. 

 A tree is evergreen when it 

 retains the leaves of one year 

 at least until after those of 

 the next season are unfolded. 

 \\'e have numerous grada- 

 tions, from our ordinary 

 " deciduous " species, which 

 are bare of leaves for five or 

 six months in the \ear, 

 through such cases as that 

 of the privet, which retains 



its leaves through a mild winter, and that of the 

 holm oak which is only stript by exceptional frost, 

 to such evergreens as the holly, or the cedars 

 and pines, that retain their needles for several 

 successive _\-ears. 



If we look at the question of leaf-fall no longer 

 geographicallv, but from the point of \iew of the 

 s^-stematic botanist, we find that the lower and 

 simpler tvpes of leaves do not fall. The primitive 

 leaves of mosses have no articulation at their base : 

 the elaborately-divided fronds of most tree-ferns 

 wither and hang their dead stalks downwards 

 from the stem : the needles of conifers wither 

 similarlv, generally after being several years on 

 the tree ; and the simple sheathing leaves of most 

 Monocotyledons have not so perfect a system of 



articulation as we find in tile Dicotyledons, especially 

 those with compound lea\'es. 



A thouglitless, unobservant conclusion would be 

 that the leaf dies and then, and consequently, falls off ; 



case. Preparations 



but this is far from being the 



may 

 it is 

 and 



begin 

 formed 

 its cells 



•->■• 



1 I If • 3 





l-roiii a f>hoto!'! ■'_ 



1- IIjL K 



Longitudinal section of a 



leaf stalk of horse chestnut 



cork layer that 



fall of the leaf almost as soon as 

 in man\' cases the leaf is moist 

 inflated when it falls. As far 

 back as 1758, Duhamel 

 ascribed the fall of the 

 leaf to a layer of tissue 

 between the stem and the 

 leaf, which remained " her- 

 baceous." i.e., capable of 

 growth, but could not stand 

 winter cold: whilst \"rolik, 

 in 1797, spoke of the ab- 

 sorption of a layer between 

 the dead and living parts 

 hut belonging itself to the 

 li\ing. In 1848 a Dr. 

 Iiiman, in a paper com- 

 municated to the Literary 

 and Philosophical Society 

 of Liverpool, described an 

 inward extension of the cork 

 of the bark and disruption 

 taking place through cellular 

 tissue external to this corky 

 la\er, from without inwards 

 ■' The provision for the 

 separation," he writes, "' being 

 once complete, it requires little 

 to effect it ; a desiccation of one 

 - ^ side of the leaf-stalk, by causing 



^ -'I \ an effort of torsion, will readily 



break through the small re- 

 mains of the fibro - vascular 

 bundles ; or the increased size 

 of the coming leaf • bud will 

 snap them ; or, if these causes 

 are not in operation, a gust of 

 wind, a heavy shower, or even the simple weight of the 

 lamina, will be enough to disrupt the small connections 

 and send the suicidal member to its grave. Such is the 

 history of the fall of the leaf. We have found that it 

 is not an accidental occurrence, arising simply from the 

 \icissitudes of temperature and the like, but a regular and 

 vital process, which commences with the first formation of 

 the organ, and is completed only when that is no longer 

 useful ; and we cannot help admiring the wonderful 

 provision that heals the wound even before it is absolutely 

 made, and affords a co\'ering from atmospheric changes 

 before the part can be subjected to them." 

 In 1859 Hugo von Mohl, the illustrious founder 

 of the cell theory, chanced to spend his autumn 

 \-acation at home, so that he observed the successive 

 fall of the leaflets and the leaf-stalk in the legu- 

 minous Gyiiiiiodaciiis canadensis with the con- 

 veniences of his laboratorv at hand. He found that 



Vi', 



m 



J. 



/y F. A'aati Clark. 

 1. 1. 



stem and the base of a 

 shewing the light coloured 

 causes leaf-fall. 



