334 FIELD AND WOODLAND PLANTS 



soil by the addition of organic matter as well as of the mineral salts 

 they contained. 



In countries where a moderate temperature is maintained 

 throughout the year, the growth of plants and trees goes on without 

 interruption, and the fall of the leaf is hardly noticeable ; for the 

 older leaves die and fall one by one, as they become incapable of 

 performing their functions for want of light, and new ones are 

 being continuously formed close to the tips of the twigs. But 

 where the growth is interrupted, either in hot countries during 

 periods of drought, or in temperate countries by the approach of a 

 cold season, the whole of the foUage is shed within a short period, 

 and new leaves as suddenly appear when favourable conditions 

 return. 



In our own latitudes, as we all know, the defoliation of the trees 

 is caused by the approach of cold weather, which decreases the 

 activity of the roots, so that the leaves become dry and lifeless. 

 It is very commonly supposed that the fall of the leaf is caused by 

 frost ; but this is not the case. The leaves are shed during the 

 cool days of autumn, even though the temperature does not fall to 

 freezing point ; but it is equally certain that the leaf-fall is accele- 

 rated by the frost when it comes, for the little moisture remaining 

 in the leaves is then frozen, rendering the structures so brittle that 

 they are easily snapped by the wind. 



The real cause of the rupture of the leaf is the formation of 

 what is called the ' separation layer.' This consists of soft, succulent 

 cells, really in several layers, which are formed across the leaf -stalk, 

 usually at the base, where the bundles of vessels passing from the 

 twig to the leaf are narrower. The walls of these cells are thin, 

 and are easily separated ; and as they extend inwards from the 

 surface all round, they break through the old cells, thus weakening 

 the junction. When the growth of the separation layer is complete, 

 it requires very little force to break off the leaf, and the process is 

 aided by the formation of certain organic acids which act on the 

 cell-walls, causing them to dissolve ; and when the leaf has finally 

 separated from the twig, it will be found that the scar left is a clean- 

 cut surface, such as would be produced by the incision of a sharp 

 knife. 



The recognition of the above facts introduces to us a difficulty 

 for which we can find no explanation : — If the leaf -fall is not caused 

 by frost, but by certain structural alterations that take place in 

 the tree itself, how are we to account for the fact that the tree 



