1906| THE SWAN SONG OF THE LEAVES. 199 
The leaves fall simply because the tree cuts them off. They 
have done their work and are no longer needed. The tree does 
not eat during its winter sleep and therefore does not need the 
food that the leaves manufacture, and though it always has to 
breathe, it can breathe through its roots and bark, just as animals 
can breathe through their skins. So it casts aside its useless 
leaves as a woman discards her summer gowns, cutting them off 
silently and without observation, but as surely and smoothly as 
they could be cut with a sharp knife. 
Besides being useless the leaves would also be dangerous if 
left on during the winter, as they would catch the snow and wind 
and thus cause the breaking of twigs and branches. Sometimes 
an early snow finds the trees unprepared, and then they are sure 
to suffer severely The snow collects on the leaves and the weight 
breaks eff great branches as thick as one’s arm. This would 
happen every year if the trees had not learned to take off their 
summer clothes ia good time. 
The cutting-off process begins when summer is at its height. 
As early as the dog days the trees begin to grow some cork cells 
between the leaf stem and the twig. This is to prevent an open 
wound when the leaf falls, for a tree can be wounded just like an 
animal. Then above the cork cells they grow a layer of another 
kind of cells. This is called the layer of separation or cutting-off 
layer, and can easily be seen on the blackberry, for instance, where 
it forms a yellowish green ring on the purple leaf stalk. There 
are three rows of cells in this cutting-off layer, and after a while 
tlhe middle one dissolves into a kind of mucilage, so that nothing 
is left to hold the leaf to the twig except some woody threads 
which pass through the cutting-off layer and the layer of cork. 
Then the cells that are left begin to swell and push the leaf stem 
from the twig, until at last a puff of wind or a frosty night snaps 
the threads and the leaf falls to the ground. 
The reason frosty nights help is because they freeze the water 
in the cutting-off layer. The resulting expansion causes the 
threads that still hold the leaf to the twig to break and as soon as 
the ice melts in the morning it falls. After frosty nights in the 
late fall, therefore, there is apt to be a great fall of leaves. That 
is why people think that the frost makes them fall. But if the 
