FALL OF THE LEAF. 359 



unharmed by the pressure of snow. Trees, bushes, and shrubs with broad out- 

 spread leaves, such as planes, maples, limes, beeches, and elms, are not capable of 

 supporting the weight of snow lying on their large leaf-surfaces. When, as 

 occasionally happens, mountain and valley are covered in snow in the autumn 

 before the leaf -fall has commenced, or when, late in the spring, to the terror of the 

 farmer, snow falls on wood and meadow after the young leaves have attained to a 

 considerable size, the devastation produced is fearful. The large-leaved shrubs are 

 pressed down and their stems broken. Branches as thick as one's arm and huge 

 tree-trunks are shattered, and in the woods quantities of maples and beeches are 

 felled, or even uprooted. Such devastation would recur every year in regions 

 with snowy winters if the leafy trees did not strip off their foliage in time, and 

 it can easily be imagined what would happen to the woods after a series of such 

 catastrophes. 



There is, consequently, a widespread idea that the autumnal leaf-fall is brought 

 about by frost. This idea is founded on the observation that when the temperature 

 in October and November falls below zero, quantities of leaves drop from the 

 branches in the early hours following the cold bright nights. Though it can 

 scarcely be denied that the fall of the leaf is in some measure connected with frost, 

 still that it is not always the immediate cause, is demonstrated by the fact that 

 when plants with leafy branches are exposed at the end of August or beginning of 

 September to a temperature below zero the leaves do not fall immediately; while, 

 on the other hand, the foliage of limes, elms, maples, cherry-trees, &c., is at last 

 stripped off in the autumn even though no frost has occurred. It can only be said, 

 therefore, as already stated, that frost is favourable to the fall of the leaf, and 

 that it hastens the commencement of the process; but not that the detachment 

 of the foliage is brought about by its sole agency. 



The detachment of the leaves from the branches is brought about by the 

 formation of a peculiar layer of cells, from the co-operation of a special tissue, which 

 has been termed the layer of separation. As a rule, leaves cannot detach them- 

 selves without the previous formation of this tissue, not even if they are exposed 

 for a long time to a very low temperature, and the sap in their cells and vessels is 

 stiffened into ice. That portion of the leaf in which the separation is to take place 

 is made up of a strong tough tissue, and the mechanical alterations produced by the 

 frost would not suffice to complete the rupture. The separation-layer, on the other 

 hand, which is formed within this tissue in one or several definite places, consists of 

 succulent parenchymatous cells, whose walls are so constructed that they are easily 

 separated by mechanical or chemical agencies, thus rendering possible a disintegra- 

 tion of the cell-tissue. The incitement to the construction of a layer of separation 

 is indeed usually the limitation of transpiration by the gradual cooling of the 

 ground, and the cessation of the absorbing power of the roots in those regions 

 which experience a cold winter. As soon as this restriction of transpiration 

 commences-and it varies very much, as shown in the previous discussion, with the 

 latitude and altitude of the region in question-thin-walled cells arise m the lower 



