04 
COURSE OF THE SAP. 
[Part II. 
has then no case; and it is a very frequent rule, 
though with many exceptions, that those leaves 
which have cases are either doubled or rolled on 
themselves, and those leaves which have no cases 
are folded over the remaining bud. 
Neither has the relative time at which the 
fruit-bud or the shoot-bud bursts any reference 
to any general chemical cause, but to the par¬ 
ticular constitution of the tree. For instance, 
the white-thorn, or May, develops its leaves and 
shoots before its flowers; the black-thorn, or sloe, 
develops its flowers before its leaves and shoots. 
And this last is perhaps the most general rule 
among fruit-trees. Many, however, develop 
flowers and shoots simultaneously. 
With regard to the death of the leaf on the 
ripening of the fruit, perhaps the only way in 
which leaves have any reference to the growth 
or to the ripening of the fruit is, that if there 
are too many leaves, their increase abstracts 
from the growth of the fruit, and their shade 
prevents the ripening of it. But do summer 
apples and pears, or do the plants which ripen 
their fruits in June and July, “yield to the 
chemical influence of the oxygen of the air,” and 
defoliate then ? If so, cherry-trees would be 
curious objects about midsummer; so would 
