182 Mr. Knight on the Parts of Trees 
trees, of old and debilitated varieties of fruit, became most 
diseased in rich soils, and when grafted on stocks of the 
most vigorous growth ; which has induced me to suspect, 
that in such cases more food is collected, and carried up into 
the plant, than its leaves can prepare and assimilate, and that 
the matter thus collected, which would have promoted the 
health and growth in a vigorous variety, accumulates, and 
generates disease in the extremities of the branches and 
annual shoots, whilst the lower part of the trunk and roots 
remain, generally, free from any apparent disease. I am, 
therefore, much disposed to attribute the diseases and debility 
of old age in trees, to an inability to produce leaves, which 
can efficiently execute their natural office ; and to some conse- 
quent imperfection in the circulating fluid. It is true that the 
leaves are annually reproduced, and therefore annually new : 
but there is, I conceive, a very essential difference between the 
new leaves of an old, and of a young variety : and in support 
of this opinion, I shall observe, that the external character of 
the leaf of the same variety at two, and at twenty years old, 
is very dissimilar ; and it therefore appears not improbable, 
that further changes will have taken place at the end of two 
centuries.* 
If these opinions be well founded, and the leaves of trees 
* The leaf of a seedling apple or pear-tree, when the plant is very young, is gene- 
rally almost wholly free from the pubescence or down, which subsequently appears 
on its under surface ; and which Bonnet and M. Mirbel, have supposed to increase 
its surface and powers. But I feel little disposed to adopt this hypothesis, having 
observed that the leaves of some new varieties of the apple, which have sprung from 
seeds of the Siberian crab, have both surfaces nearly equally smooth; and that these 
varieties grow faster, and bear heavier crops of very rich fruit, than any others, without 
being exhausted or injured. 
