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species in bygone ages, and under different circum

stances.

THE FALL OF THE LEAF

Having traced up the leaf from its earliest rudiment, it remains, in conclusion, to say a few words about its final stages.

The fall of the leaf (41) is by no means a simple process or a mere case of death. In the first place, the chlorophyll-substance is too valuable to be lost and thrown away. Sachs' has satisfied himself that it passes out of the leaf, down the petiole, and thus migrates into the persistent parts of the plant, and is stored up for future use. With the general disappearance of the cell-contents the protoplasm and cell nuclei are dissolved, the chlorophyll corpuscles lose their normal outlines, the starch disappears, and the colouring matter changes, leaving in many cases a large number of small, bright yellow granules, to which the yellow tints of autumn are due. In many cases the cell-sap becomes bright red. Sachs has been able by chemical examination to follow the materials (and especially the starch), and also the most valuable chemical materials (especially the potash and phosphoric acid), down the petioles, into the twigs, where they are preserved, evidently to nourish the growing buds of the following spring.

While these changes are taking place a new layer of cells is formed across the base of the petiole (fig. 340).

1 Flora, 1863, p. 300; Phys. of Plants, tr. by H. M. Ward, p. 318.

These cells eventually adhere less and less closely together, so that at last the slightest touch or the gentlest wind brings the leaf to the ground. That this is not a mere case of death of the leaf, but, on the contrary, is a vital process, we may easily convince ourselves by breaking a branch during summer.

In

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FIG. 340.-LONGITUDINAL SECTION THROUGH THE JUNCTION OF STEM AND LEAF OF THE HORSE CHESTNUT IN AUTUMN.

a, parenchyma forming the outer cortex of the twig; b, external cork layer of the same; c and e, parenchymatous tissue at the base of the leaf-stalk; separating layer, the continuity of which is already broken in the upper part.

that case it is killed, the new layer of cells is not formed, and the leaves remain firmly attached to the twig.

It is also of interest to notice how plants utilise the fact of the temporary nature of their leaves. Leaves share with the bark (also transient) the functions of the excretory organs of an animal. In their cells are de

posited the waste products of the chemical life-processes, to be got rid of when the leaf falls or the bark peels. A leaf which has served its time and then been dropped contains quantities of calcium oxalate, resinous and gummy matter, and, like the bark also, alkaloids, such as nicotin, thein, and other nitrogenous excreta.

Climate has, naturally, a great influence on the length of time during which leaves remain attached. Where autumn is prolonged the leaves also last long. Thus, for the neighbourhood of Naples, excepting when winter sets in unusually early, Tenore quotes the end of November for the leaf-fall 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, and the Cherry is evergreen. 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. Trees do not lose their leaves at the same time: not only are Oaks and Elms still green when the Horse Chestnuts and Limes are almost bare, but some individual trees retain their leaves longer than others of the same species. Apart 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 originally from somewhat different climates. A plant may have contracted the habit through long generations of shedding its leaves early or late, as the case may be, and

retain the habit under changed conditions. De Candolle explains early leaf-fall in some cases by introduction from an extreme climate where it would be necessary, and instances the Walnut, introduced from Armenia.

Another question is, Do the plants which come into leaf early in spring lose their leaves early in autumn, and are the late leafers also tardy as regards defoliation? Or are the early leafers late to lose their leaves? The hawthorn hedges are among the first to show signs of green in the spring, and among the last to lose their foliage in the autumn. The Horse Chestnut buds have burst when the Oak and Ash are grey as at midwinter, but the latter retain their leaves the longer. We must remember, too, that the Ash gets its flowering over before leafing, while in the Chestnut the leaf-buds are the first to expand. To try and settle this point De Candolle made a résumé of four years' observations in Belgium, in which twenty-eight species or varieties were concerned. He made four lists: A, the early leafers; B, those with an early leaf-fall; C, late leafers; D, those where the fall is late. Each list contains about thirteen names. Two, Spiraea sorbifolia and Lilac, are common to A and B; that is, out of thirteen early leafers only two lose their leaves early. Four are common to A and D: leaf early but fall late. Two, Walnut and Gleditschia ferox, occur in both B and C, having an early fall, though leafing late; and four are late in leafing and have also a late leaf-fall. It is therefore evident that the time of leaf

fall does not depend on the time of bursting of the buds. De Candolle also quotes observations on individual trees. In many species, like Horse Chestnut, Beech, Elm, &c., trees growing side by side, planted at the same time, and subjected to the same external conditions, neither come into leaf nor lose their leaves simultaneously. These facts of idiosyncrasy, he says, are constant. They recur in the same manner year after year, even when the differences in time are only three days. Of three old trees in the court of a country house at Frontenex, the owner stated that for years they followed the same order of succession at both epochs : those which were first to leaf were also the earliest to lose their leaves. A Chestnut in the gardens of the Tuileries from its habit of early leafing is known as the Vingt Mars; and M. Henri Vilmorin states that, compared with the other Chestnuts in the gardens, it keeps its leaves beyond the mean time of leaf-fall. M. Vilmorin also observed thirty-four Horse Chestnuts, 150 years old, in a plantation on his own property at Verrières in 1876 and 1877. He found that those which were the first to lose their leaves in 1876 were in the same category next year,

Six of those which

with slight differences as to order. were the first to put out leaves in spring were late to lose them in autumn, while five others were early in leaf and early in losing leaf. He concludes that the two phenomena have no regular relation. Similar observations on the Beech and Elm led to the same conclusion.

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