440 
PHYSIOLOGY. 
BOOK II. 
to the elevation of temperature, and the dampness of the air. 
This fact must be familiar to all who are acquainted with 
gardens. In the hot dry weather of a summer’s noon, flowers 
either become scentless, or at least lose a large proportion of 
their usual fragrance ; and, in walking through a wilderness 
of the most sweet-smelling plants, we find little sign of their 
odour, unless they are bruised or trampled upon. But if a 
heavy shower should come on, all will be changed in an hour’s 
time ; every leaf, every flower, will emit its peculiar odour ; 
the Musk Plant (Mimulus moschatus) will fill the air with its 
singular scent, and it will be obvious that the addition of 
moisture to the air has produced a total change in the action 
of the odoriferous organs of plants. 
The same phenomenon is daily repeated in the driest 
days of autumn. Those only who are accustomed to take 
their early walks abroad can have any idea of the difference 
between a richly stored garden early in the morning and at 
noon. When the sun has dried the air, and has been beating 
for some time upon vegetation, ill able to bear his action, in 
consequence of the dryness of the source from which it 
draws its means of compensating for evaporation, however 
beautiful a garden may still remain, it cannot be compared to 
the same place before the dew has dispersed ; when every 
herb, tree, and flower is pouring forth a stream of the most 
varied and delicious fragrance ; when the air is impregnated 
with the most delicate balsamic odours ; and when all nature 
seems as if offering up incense in gratitude for the refreshing 
powers of darkness and of dew. Let any one, for example, 
visit a thicket of Cistuses at noon, and again the next morn- 
ing, and the difference will be exceedingly apparent. To 
what cause this is owing is unknown ; possibly the effect of 
dryness and excessive heat may be to close the stomates, and 
to contract the tissue of plants, thus rendering it difficult for 
volatile matter to pass through their cuticle : it may also act 
by depriving them of the necessary proportion of water re- 
quired to enable them to perform their functions of secretion 
and assimilation, and thus arrest for a time the elaboration of 
the fugitive principles upon which fragrance depends. While, 
however, dew and showers, with intervals of bright light, are 
