The Individual Plant 



One often sees this, e.g,^ in herbarium specimens 

 which are being dried. Even in spite of heavy weights 

 upon the blotting-paper, fresh whitey-yellow buds are 

 put out by the younger parts of a Sedum or Sempervivum, 

 and a Httle anaemic-looking shoot goes on growing, 

 whilst the older leaves and stem are dead and rapidly 

 drying up. But this occurs in other plants also. When 

 Elder, Philadelphus, and Robinia branches are allowed 

 to wither, they will keep the younger branches and 

 leaves fresh and green, sacrificing the older leaves, 

 which soon die and drop off. An Ipomoea, for instance, 

 flowered during this process of withering.^ 



In such a case, however, it is most probably the keen 

 competition between the thirsty leaves for water that 

 results in the weaker ones being killed by thirst. 



Suppose that a leaf is injured by, e.g., an insect such 

 as the roseleaf wasp, which cuts out of it a little circle 

 of leaf tissue, then clearly there is a danger in the open 

 circular wound. The loss of water by evaporation 

 might very soon dry up the leaf, and what is even more 

 perilous, one of the many dangerous fungus spores 

 might attack the freshly exposed inner tissue and destroy 

 not only the leaf but possibly the whole tree, for the 

 poisonous secretions of such a fungus would very likely 

 pervade the entire plant. But leaves are well able to 

 deal with such an emergency as a cut or bruise. 



Immediately this happens, the living cells, some little 

 way in from the wound, begin to divide and form a 

 peculiar mass of cork-like tissue filling the space be- 

 tween the epidermis of the upper surface and that of 

 the lower. Everything outside this cork-like layer dies, 

 because, of course, neither water nor food material can 

 pass through its closely set cells. So by a kind of 

 surgical operation the dead and injured part is cut out 

 and soon withers and drops off. 



Professor Blackman and Miss Matthaei made many 



153 



