232 FALLING LEAVES. 



and their broken-off ends at first project when the petiole is 

 actually removed. And it is to these the peculiar marks are due, 

 which are seen in every " leaf-scar." 



When the leaf-stalk drops off, carrying the Absciss layer with it, 

 there is left behind at first, apparently, some greenish parenchyma 

 cells on the outside. But the foregoing section shows that under 

 this veil of parenchyma there is already prepared a layer of 

 periderm, with its plane extending beneath the whole, except through 

 the Fibro-vasal bundles. But nowit begins to develop through these 

 also, that is, right across the roundish area left bare. Conse- 

 quently the outer cells just mentioned dry up, and ultimately a 

 completely closed layer of cork is formed over the recently exposed 

 tender spot. This increases somewhat in thickness until the 

 branch is effectually protected against the coming storm and cold of 

 winter. Thus the tree remains adapted to its environment, until 

 once more spring draws on, and the daily increasing amounts of 

 heat and light indicate that it is safe and profitable to put forth 

 again buds and leaves. Then recommences that fixation of carbon, 

 with evolution of oxygen, whereby the tree itself is nourished and 

 the healthful balance between the vegetable and animal world is 

 so wondrously maintained. It seems to me impossible to attribute 

 the marvellous inter-working of all these processes, reasonably, to 

 mere fortuity. 



EXPLANATION OF PLATE XV. 



Fig. 1. — Medial vertical section through insertion of Petiole in 

 Autumn before fall of leaf. jEscuIus Hippocastanum — 

 showing — a, ^, epidermis ; h, i, tibro-vascular bundles ; c, 

 cortical parenchyma ; d, absciss layer ; e, periderm forming ; 

 /, parenchyma ; h, medullary rays. 

 ,, 2. — Transverse section of Ribes nigrum, showing Cork formation 

 of first year :— «, epidermis ; h, cortical parenchyma ; 

 c, phellem (cork from cambium) ; d, phellogen (cork cam- 

 bium) ; e, phelloderm ; /, phloem (bast) ; g, periderm. 



