1^6 



IRISH GARDENING. 



How and Wl 



IV do 



I. 



ca\cs 



Fall 



THK fall ol' I 111- li'.iT ill Auiumii is a pliciionu-non 

 woll known to every one. Hut the reason wliy 

 our deciduous trees and shrubs shed tlieir 

 leaves after the seasons work is over is understood by 

 relatively few. It is caused by a deliberate act of the 

 plant itself— in other words, it is the result of life and 

 not of death. .\ dead bough will not and cannot shed 

 its leaves. An excellent account of leaf-fall is given by 

 Professor Boulger in a recent issue of Kmnvlcdirc which 

 we take the liberty of reproducing : — 



There is no element, he says, which contributes more 

 to the ditTerence of the landscape as we travel from the 

 Kquator 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 evergreens, a 

 characteristic which extends northward in the more 

 insular moist climates of coast and island regions, 

 notably exemplified by the flora of Japan. In the 

 Cooler Temperate Zone the predominant trees are 

 dicotvledonous angiosperms, the " broad-leaved trees " 

 of our foresters, with smaller, thinner leaves than those 

 of the jungle, lighter in tint, producing a less 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-leaved" conifers, mostly 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. We have numerous 

 gradations, from our ordinary "deciduous" species, 

 which are bare of leaves for five or six months in the 

 year, 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 stripped by exceptional frost, to 

 such evergreens as the holly, or the cedars and pines, 

 that retain their needles for several successive years. 



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

 graphically, but from the point of view of the systematic 

 botanist, we find that the lower and simpler types of 

 leaves do not fall. The primitive leaves of mosses have 

 MO 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 coni- 

 fers wither similarly, generally after being several years 

 on the tree ; and the simple sheathing leaves of most 

 -Monocot vledons have not so perfect a system of articu- 

 lation as we find in the Dicotyledons, especially those 

 with compound leaves. 



A thoughtless, unobservant conclusion would be that 

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

 this is far from being the case. Preparations may 

 begin for the fall of the leaf almost as soon as it is 

 formed ; and in many cases the leaf is moist and its cells 

 fairly 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., capaple of growth, but could not stand 

 w'inter cold ; whilst Vrolik, in 1797, spoke of the absorp- 

 tion of a layer between the dead and living parts but 



belonging itsi-if to the living. In i.S4Sa Dr. Inman, in 

 a p.iper comnuMiicaled 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 layer, from without 

 inwards : — 



•' The provision for the separation," he writes, " being 

 once complete, it requires little to effect it ; a desicca- 

 tion of one side of the leaf-stalk, by causing an effort 

 of torsion, will readily break through the small remains 

 of the fibro-vascular bundles; or the increased size oi 

 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 vicissitudes oi tem- 

 perature 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 covering 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 vacation at 

 home, so that he observed the successive fall of the 

 leaflets and the leaf-stalk in the leguminous Cijmnocla- 

 dus canadensis with the conveniences of his laboratory 

 at hand. He found that a layer of cork already ex- 

 tended through the cellular tissue at the base of the 

 petiole in September. Immediately above this a layer 

 of cells had become brown (suberised); and, separated 

 from this by two or more rows of the ordinary colour- 

 less polyhedral cells of the leaf-stalk, what he termed 

 the separating, or " absciss," layer originated. This 

 only formed between the 4th and the 15th of October, 

 extending across the stalk from the inner or axillary 

 surface, and contained in its cells protoplasm and 

 starch-grains. It is, in fact, what we now term 

 "secondary meristem." Von Mohl only recognised tvvc"> 

 layers of cells in the absciss-layer, which he believed to 

 split apart, while he thought that the fibro-vascular 

 bundles were broken mechanically by the weight of the 

 blade and the strain of wind and rain. He perceived, 

 however, that the fall of the leaflets between the loth 

 and 2oth of October, and the subsequent fall of the 

 petioles was independent of the cork-layer formed at 

 least a month before. This cork-layer, in fact, is not 

 formed in advance in those ferns which are deciduous 

 in beech, elm or most oaks. Von Mohl also noticed 

 that when leaves fell suddenly, after an autumn frost, a 

 thin layer of ice had formed in the delicate sappy cells 

 of the absciss-layer, torn cell-walls evidencing the 

 violence of the disruption. 



In 1863, Julius Sachs traced the gradual removal of 

 the contents of the leaf-cells. The protoplasm and 

 nuclei are dissolved, the chlorophyll granules become 

 disintegrated, the starch disappears, leaving only the 

 few yellow granules, or the reddened cell-sap, which 

 produce our autumn tints ; while starch, potash and 

 phosphoric acid travel down the leaf-stalks to be stored 

 up in the twigs, and only the waste or end-products of 



