CHAP. XIV. 
DIRECTIONS TAKEN BY ORGANS. 
415 
towards the sky, and of the lower towards the earth. This 
disposition is so powerful, that, if the usual direction of a leaf 
be inverted, the petiole will twist so as to enable it to recover 
itself. This phenomenon has been noticed by Bonnet, whose 
explanation has been already given, but which is obviously 
inadmissible. There is always a natural difterence between 
the two faces of the leaf : the upper is always the more 
deeply coloured ; a difference which will be found constant 
in all cases. The face with the deeper colour turns towards 
the sky or light, and that with the weaker colour towards the 
earth or obscurity ; and this is so constant a law, that it will 
be found that if the 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 being inverted. This may be 
seen in many grasses, but best 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 colouring 
matter deposited in the parenchyma lying below the epider- 
mis. Thus the whiteness of the flowers of plants is not 
dependent upon the absence of colour, as in 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. 
