84 GENERAL BOTANY 



no change takes place in their vertical distance one 

 from another on any of these trees. 



4. Cork is formed not only in the bark of trees 

 and shrubs, but also across the stalks of the leaves, 

 at the base where they join on to the branch. When 

 it has formed completely across, the leaf necessarily 

 dies and falls, breaking off at the corky layer. So 

 that the wound or scar so made is covered at once 

 by the cork, and loss of water from the exposed 

 surface is prevented. This is why leaves fall off 

 so cleanly with their stalks, and do not leave a 

 ragged broken stem. 



Palms, on the other hand, do not form cork (just 

 as they do not increase in thickness) and so when 

 the leaf dies it breaks and leaves a ragged, untidy 

 leaf -base still on the stem. It is only later on that 

 the leaf-base becomes detached and leaves the stem 

 clean. 



Cork is also formed wherever soft living tissue is 

 exposed by a wound. If a leaf be cut a thin brown 

 layer forms and prevents loss of water from the cut 

 surface. The same happens with fruits and tubers. 

 If a potato is cut, cork forms over the cut surface ; 

 and if it be pricked with a needle a cylindrical layer 

 forms round the wound and prevents loss of water. 



The great difference that even a v r ery thin layer of 

 cork makes can be seen very easily in this way. 

 Take two young potatoes, the smaller the better, as 

 nearly as possible the same size. Weigh them, and 

 weigh again in a day or two. Provided that the skin 

 is unbroken very little difference will be found in their 

 weights. But remove the skin from one ; weigh it, and 



