2 ;6 ROUND THE YEAR 



out in sections by the naked eye, especially if iodine 

 solution, which gives a characteristic blue colour to 

 starch, is applied. It forms gradually, extending across 

 the leaf-stalk from without inwards. 



The separating layer consists of growing tissue, 

 which absorbs whatever nourishment it can draw from 

 the neighbouring cells, and displays a short-lived 

 activity. New cell-walls, parallel to the direction of 

 the layer, soon appear, and in a few days or even in a 

 few hours after its first appearance in a recognisable 

 form it becomes divided into three tiers of cells. Each 

 cell is very thin or low, in proportion to its length 

 and breadth. When the right moment comes, the 

 middle tier of cells breaks down, the cell-walls being 

 most likely converted into a kind of thin mucilage by 

 a change of which many other examples could be 

 furnished, and thus the cellular tissues of the leaf- 

 stalk are severed. 



Since Mohl's discovery further investigation has 

 brought to light not a few interesting details. Van 

 Tieghem and Guignard l have pointed out the signifi- 

 cance of certain peculiar brown cells, previously seen 

 by Mohl, which stretch across the leaf-stalk near the 

 separating layer. They are cells which have become 

 lined with cork, and thus rendered impervious to 

 water and watery fluids. Before the leaf is shed the 

 cellular tissues have their supply of water and sap cut 

 off. The vessels, however, still remain open, for they 

 are wanted to discharge whatever useful fluids the 

 worn-out leaf may still contain. After the leaf has 

 fallen, the vessels too may be sealed by corky sub- 

 1 Soc. Bot. de France^ torn. 39 (1882). 



