The Individual Plant 
One often sees this, ¢.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 little anzmic-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 Ipomeea, 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, ¢.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 
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