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president’s address. 
too rapid dissipation ; moreover, a change of temperature may, by 
expansion, or contraction, exert a pressure on the vesicles which 
contain the volatile scent. 
The fragrance of flowers is, however, affected by light as well as 
by heat. We have an illustration of this in an article in the 
Journal de Pliannacie, 1827, on the effect of the sun’s rays upon 
the flowers of Cacalea septentrional is : “When the sun shines 
upon the flowers of this plant they are odoriferous, but when the 
sun’s rays are interrupted by artificial means — that is, by interposing 
the hand- — their odour quickly disappears, but their fragrance as 
quickly returns when the shade is removed.” Many plants are so 
sealed by their closing petals that it is easy to see that perfume 
cannot escape until they re-open ; though with others the scent is cut 
off without anyjapparent change in the flower; but it may be that 
with such there is a slight change in the position of the petals. 
The Eose is one of the few flowers which have scented petals, 
hence its use in making pot pourri; it possesses also the only scent 
on which, when extracted, direct sunshine has no deleterious effect. 
The time at which flowers re-open and re-commence shedding 
their perfume often agrees with the time of day at which the buds 
first opened ; in fact, there are many cases in which the clock rather 
than the barometer appears to regulate the expulsion of odours. 
The behaviour of one of the night-blooming Cactacece often quoted, 
may be referred to as bearing on this subject. The flowers are 
fragrant only at intervals, giving out puffs of odour every half-hour, 
from eight in the evening till midnight, the first puff of odour 
being accompanied by a rapid movement of the calyx. 
We may next enquire as to the advantages gained by the plants 
from their perfumes. The chief benefit is the allurement of insects, 
for the transference of pollen from the stamens of one flower to 
the pistil of another of the same species. 
While some plants arrest the attention of insects by their brilliant 
colours, others attract them by their sweet odours, and in many 
plants both influences are at work, as in the Eicotiana of our 
gardens, whose conspicuous white flowers, after being furled during 
the sunlit hours of day expand in the evening, and at the same 
