264 OF THE RELATION OF VEGETATION TO SEASONS. 
it appears from experiments that heat applied to the branches alone, 
is quite sufficient to determine and maintain all the phenomena of 
growth. 
“ Once set in action, the branches of a tree go on growing according 
to the laws which have now been explained. They and their leaves, 
by degrees, gain their full growth; bark and wood separate, and cam - 
bium is deposited between them ; the leaves decompose the Jiuid they 
receive, send their jibres down within the substance of the branches, 
gradually secrete the substance peculiar to each peculiar species, and 
transfer them to the bark ; and, finally, becoming clogged at every pore 
by the earthly and carbonaceous matters that are deposited during the 
processes of digestion and evaporation, cease to act efficiently as leaves. 
ee In this state, they are principally protectors of the young buds in 
their axils. If the latter have been formed very early, they are so far 
advanced in their growth by the middle of summer, that they have 
already arrived at the same state as later-formed buds will be in at 
the commencement of another spring. Acted upon by the temperature 
of the season, they develope and call into play the same class of pheno¬ 
mena as took place in the beginning of the spring ; the sap which had 
become languid as the leaves became impotent, is again stimulated to 
a rapid movement, and is secreted anew in increased quantity. This 
is indicated by what gardeners call the running of the bark—that is to 
say, the bark and wood of exogens separate spontaneously as in the 
spring, depositing a layer of cambium between them. Thus are formed 
what are called midsummer shoots, which only occur in plants which 
bud very early in the spring. 
In the course of the autumn, the increased and prolonged heat and 
drought complete the destruction of the leaves, which had already 
begun to languish; and their vital actions are destroyed by the quan¬ 
tity of foreign matter with which their cells, their stomates, their 
vessels, and their intercellular passages are filled, and they drop off. 
At this time a plant is nearly exhausted of its fluid sap, the watery 
portion of which it had exhaled during the summer and autumn ; all 
the parts are dry and solidified, so as to suffer little from evaporation; 
and the roots themselves, having for some time been but feebly in 
action, are firm and not liable to be easily broken; every thing is in 
a state of languor, and prepared to renovate the enfeebled powers of 
the plant by the slow and gradual absorption of fluid during the 
winter. 
“ It is in the autumn, then, that both theory and practice direct us to 
transplant trees. At that season every circumstance concurs to render 
