ON THE SEASONAL GROWTH OF PLANTS. 
447 
zones, plants are regularly seasonal; that is, they grow, blossom, and 
yield seeds in one season, and rest in the other. And even in the hot¬ 
test parts of the torrid zone, although there is nothing like summer and 
winter, yet there are changes of weather which affect plants in a similar 
manner. There is the dry season, especially towards the end of it, in 
which a great majority of plants cease to grow, and even shed their 
leaves ; and many wholly disappear, withdrawing themselves entirely 
from the light and heat of the sun, and remain in a torpid state till the 
dry heat is over. This is followed by the rainy season, which recovers 
all vegetation ; the foliage and flowers of bulbs and tubers spring forth 
from their dormitories, the seeds of herbs are awakened into life, and the 
shrubs and trees all put on a new livery. 
In latitudes where the regular seasons of summer and winter are con¬ 
stantly experienced, the development of plants in the one, and their 
torpidity in the other, are common occurrences, and every day before 
our eyes. And although it be perfectly obvious that the changes of 
temperature are the principal and ostensible causes of vigorous action, 
and rest alternately, there are instances of particular plants, which 
though they obey the revivifying influence of the vernal season, stop 
long before that degree of heat which first excited them to action ceases 
in the autumn. Of this fact the common ash tree (Fraxinus excelsior) 
is an instance : although it is one of the latest in coming into leaf in the 
spring, it is one whose summer growth is quickly over, rarely extending 
its shoots after the month of July; and sometimes its growth ceases 
soon after Midsummer. 
This circumstance plainly shows us that heat alone is not the agent 
which can cause vegetable development to be progressive, and that it is 
not the southern declination of the sun, nor the diminution of his power 
which arrests the growth of some plants, but rather that there is some 
constitutional law which regulates the growth by impulses ; certain 
portions of the organisation being developable, and when this is com¬ 
plete a pause takes place for a longer or a shorter time. 
Most of the plants in the temperate zones make annual pauses. 
Their visible growth and the annual divisions of their woody structure 
show this very evidently. Some few there are which make a double 
pause ; a short one at Midsummer, and a long one during winter. The 
oak always, and the horse-chestnut frequently, present this Midsum¬ 
mer rest, succeeded by what is called the Midsummer shoot. 
In the equatorial regions the pauses or cessations of growth are 
caused, as already observed, by the alternations of the dry and rainy 
seasons; and although in constantly humid or marshy situations, there 
