PERIODICITY IN VEGETATION. 133 
Polyanthus, a variety of the Primrose, usually accompanies the 
Snowdrop, the Crocus, the Daffodil, the Crown Imperial, and all 
the spring flowers, both early and late. It is well known that 
the duration of flowers is very variable, as much so as the plants 
that produce them, although the range be smaller; but it is not 
so commonly known that temperature or atmospheric influences 
generally accelerate and retard the flowerig of plants, as they 
prolong or shorten the duration of the flowers themselves. In 
this very summer (1855) the Hawthorn, which, on an average of 
years, flowers in the first or in the second week of May, blos- 
somed with the Elder, which seldom flowers before midsummer. 
(See ‘ Phytologist ? for September.) Few are unacquainted with 
the fact that certain plants flower in certain seasons; some 
early, some late. We do not look for snowdrops and daffodils at 
midsummer, nor for roses at Christmas. Each and all sorts of 
plants have their seasons or periods of manifesting their vitality, 
or activities of different kinds, and also their seasons of repose, or 
decay and death. This is what is understood by the periodicity 
of plants, and is observed by all; but the differences of latitude, 
of exposure or shelter, or soil, or even the idiosyncrasies of cer- 
tain plants of the same species and in the same locality are not 
generally noticed. Hence arise the great discrepancies in the 
times or periods when a given plant is in flower ; for over a range 
of ten degrees of latitude it is impracticable to attempt more 
than an approximation to the average or mean time of a plant’s 
flowering, or of the duration of its flowers. 
Temperature and moisture have a very powerful influence on 
plants, either in retarding or accelerating their periods of flower- 
ing. This fluctuation is as much as a month in March and 
April—probably a fortnight or three weeks in May, and at least 
a week at midsummer. If local circumstances—viz. soil and 
exposure—were uniform, we could, indeed, approximate very 
closely to the period when these common plants flower, by com- 
paring the average temperature of past years and months with 
the temperature of the year in which we wanted to know when 
a given common plant might be expected in flower; but soil and 
situation are susceptible of so many modifications, that a close 
approximation is not to be expected. In the early part of the year 
rain delays the flowerimg of plants; at this time the earth is 
usually sufficiently moist, and plants only want heat to accelerate 
