132 PERIODICITY IN VEGETATION: 
in all plants, there may be observed certain changes which are 
ascribed to the combined influence of light and temperature, or 
to one or other of these agents. What has been fancifully termed 
the sleep of plants appears to be owing to one or both of these 
causes. In the latter part of the day, or towards evening, legu- 
miniferous plants close their leaves; that is, the leaflets approach 
each other, as if for mutual protection. This phenomenon is ob- 
servable in the Trefoils, and in other trifoliate plants; the two 
lateral or basal leaflets approach face to face, and the terminal 
one is applied to the edges of the lower ones. Certain blossoms 
unfold their petals in the morning and shut them up before noon, 
as the Goat’s-beard, and some other compound flowers. Others, 
as the Evening Primrose, Silene noctiflora, etc., open their showy - 
flowers about sunsetting. Some, as the Poor Man’s Weather- 
glass, Chickweed, and many plants of the Caryophyllaceous order, 
expand their little blossoms when the atmosphere is dry; and in 
consequence a fine day is anticipated when these plants open even 
to a cloudy or perhaps mizzling sky. These facts prove that cer- 
tain species have idiosyncrasies or characters peculiar to them- 
selves. The decrease of heat acts upon the tissues of Trefoil, and 
of many other plants, and the effect of this action is, as above 
stated, the converging of the leaves. The same causes accelerate 
the opening of the Evening Primrose. Some plants, as the Dande- 
lion, open when the air is moist, as it usually is in the morning; 
the Pimpernel closes when moisture is present in the atmosphere. 
The same atmospheric causes produce contrary effects on different 
objects. Diurnal changes appear in some plants, which are al- 
ways affected by the rotation of day and night. In other plants 
there may be a change, though it is not so manifest im the ex- 
ternal organs as it is in those of the plants above mentioned. 
Duration of Flowers——Many blossoms last only for a single 
day,—as those of Hemerocallis, the Yellow Lily, or ‘ Beauty of a 
day,’ the Evening Primrose, and the Poppies. The flowers of 
the Tigridia pavonia fall off immediately after expansion. In 
bright warm weather some of the petals (parts of the flower) fall 
off before the rest are fully expanded. In moist or cloudy wea- 
ther only do these flowers remain for any considerable portion 
of a day. On the other hand, some plants are in flower for weeks. 
The common Primrose is frequently in flower during the months 
of January, February, March, April, and May. In gardens the 
