450 ON THE SEASONAL GROWTH OF PLANTS. 
these cases it is very clear that atmospheric heat, abstractedly con¬ 
sidered, is not the sole agent in vegetable development, otherwise the 
growth, both as to vigour and duration, would always be equal to the 
intensity of the inciting cause ; and, according to this rule, the develop¬ 
ment at Midsummer, when the sun’s heat is usually greatest, should be 
more luxuriant than at any other time of the year. This, however, we 
know is not the fact; and, therefore, we can only rationally attribute 
the stagnation of growth at any period of the warmest season to some 
constitutional peculiarity of the plants respectively. 
In regard to annual, biennial, and perennial herbs, their development 
is easily comprehended. They grow up, flower, ripen their seeds, and 
die entirely in the first and second years, if annuals or biennials, or die 
* • 
down to the ground if perennials. The annual growth of a tree or shrub 
having a permanent head is very different. These have a certain num¬ 
ber of shoots, leaves, flowers, and fruit to mature, and moreover a new 
layer of wood and a new liber to distend and maintain. This distension 
is effected by the rarification of the sap stored in the cellular and vas¬ 
cular membranes inducted in the last or other bygone years, and this 
rarefaction and expansion is supplied by water and other elemental food 
absorbed by the roots from the earth, and by the stomata or pores of the- 
bark and foliage from the air. 
Now, if there be not a sufficient supply of food collected from these 
sources for the amplification of the shoots, leaves, flowers, fruit, and’ 
new layers of wood and bark, the full expansion of the parts cannot be 
complete, and of course the growth will cease early in the summer, or 
the expansion of the different members will be both slow and dimi¬ 
nutive. But, on the other hand, if the plant be healthy, and can supply 
itself with a full quantum of food, the expansion of every part will be 
more ample, and the growth longer continued. But even uuder those 
favouring circumstances it does not appear that the growth will be con¬ 
tinued longer than the period necessary for completing the previously 
prepared portion of the organisation, unless art interferes to derange 
the process. For instance, if an annual plant be cut over early in the 
season, and before either flowers or fruit are produced, other shoots 
will be produced to complete the purpose of the plant; and if these be 
again cut off, this mutilated being, whose period of existence is pre¬ 
destined by nature to the space of only a few months, may be kept 
growing for twelve or more : this happens with many of the grasses. 
So if a shrub, a rose, or a rose acacia, be pruned back as soon as they 
have yielded their first flowers, they will shoot and flower again in the 
autumn. The like takes place on the development of trees, and fruit 
trees particularly : if their first shoots or flower-buds be destroyed, others- 
