342 
PHYSIOLOGY. 
BOOK II. 
viously inadmissible. There is always a natural difference 
between the two faces of the leaf: the upper is always the 
most deeply coloured ; a difference which will be found con- 
stant in all cases. The face with the deepest colour turns 
towards the sky or light, and, with the weakest colour, towards 
the earth or obscurity ; and this is so constant a law, that it 
will be found that if that surface of the leaf which is naturally 
inferior is more deeply coloured than the superior, the petiole 
will be twisted round by the greater affinity of the lower sur- 
face for the light, which will thus become uppermost, the leaf 
presenting the appearance of an inverted leaf. This may be 
seen in many grasses, but not in Zea Mays, Triticum repens, 
and Agrostis rubra. Hence it is to be concluded, that the 
upper surface of the leaf is not turned towards the heavens 
merely in consequence of its quality of being the upper sur- 
face, but because it is generally the most deeply coloured. 
The same law influences the directions of the petals, in 
which the upper surface, — that which is turned towards the 
heavens, — is always the most highly coloured : this, indeed, 
is sometimes not very apparent, but is nevertheless constant. 
Even in white petals, — such, for example, as those of Lilium 
album, — the upper face will be found of a dense but brilliant 
white, while the lower is of a much paler hue. The white 
colour of the petals, Dutrochet proceeds to remark, like all the 
other colours of plants, is due to a particular kind of colour- 
ing matter deposited in the parenchyma lying below the 
cuticle. Thus the whiteness of the flowers of plants is not 
dependent upon the absence of colour, like the roots and 
etiolated stems : in the former a white colouring matter exists ; 
in the latter the whiteness is caused by absence of colour. 
Some apparent exceptions to this law, — such as the outside 
of many monopetalous flowers being paler than the inside, as 
in Digitalis purpurea, Fritillaria latifolia, and others, — Du- 
trochet thinks may be explained thus : — These cases, no doubt, 
are due to the tendency of the less coloured part to avoid the 
light, which is manifested by bearing down the flower so as to 
approach the seat of obscurity as nearly as possible : all such 
flowers being always nodding. This tendency is aided by 
the weakness of the peduncle, which seems to have been 
