12 The Structure of Leaves. 



up as to be quite unable to peiforra their appointed duties. But 

 it may be asked, why do not e%'ergreens suffer in this way ? The 

 answer is, that the leaf of an evergreen does die every year (except 

 in a few cases, as in the pines, which however lose their leaves also 

 after two or three years), but they do not lose their colour or fall 

 off" until the new leaves displace them in the following spring. 

 If then the leaf dies, it may be asked, " why does it not fall?" 

 This leads us to distinguish between the "'death'' and the ''\fair'' 

 of the leaf. That they are two distinct things is ])roved by the 

 fact that the leaf often falls before it is dead as well as afterwards. 

 The fall of the leaf is owing to a vital process, which begins at the 

 very moment the stalk of the leaf is formed. Exactly at the point 

 where the stalk joins the stem of the tree a membrane gradually 

 grows in a transverse direction through the stalk, beginning from 

 the epidermis and growing inwards, which at length meets in the 

 centre of the stalk. The leaf would now fall at once if it were not 

 for some woody bundles (partaking of the nature of veins in the 

 blade of the leaf), which the membrane does not cut through. Now 

 if these are very strong — as they are in evergreens — the leaf 

 will not fall until the new leaf, forming underneath the old, bursts 

 them asunder. If, however, they are but slender, a gust of wind or 

 a slight desiccation will snap them, or even the weight of the blade 

 may be sufficient to effect this. Now herein there is, perhaps, the 

 most striking point in nature's economy with regard to leaves For 

 if this membrane did not grow the effects of the fall of the leaf 

 would be very serious to the tree, indeed, in many cases fatal ; for, 

 in the first place, the leaf would not come off" clean" as if cut with 

 a knife, as now, but would tear off pieces of the tree along with it ; 

 again there would be an open wound, whence the sap would bleed 

 out and weaken the tree. As it is, the membrane heals the wound 

 before it is made, so to say, and only a scar is formed, which may 

 be seen on any tree, except of course evergreens, in the autumn. 



Thus then the fall of the leaf is not due to change of tempe- 

 rature, or to any accident, as it were, but to a regular process, 

 which commences with the first formation of the organ, and is 

 completed only when it can be of no further use. 



We have now traced the history of the leaf, first to its full 

 growth and then to its fall, and we have seen the method and 

 reason in both, but this is not all, the leaf is not useless even after 

 death. Nature does not waste any single atom that she has created. 

 AVhen the leaf has fallen it rots, which is merely a process of 

 decomposition, or the separation of elements before combined ; at 

 this time, then, the particles of earth that have been stored up in 

 each leaf during the summer, drawn up from the earth, are returned 

 to the earth, and not without interest for the loan, for the other 



