Mr. E. C. Nourse on the Colours of Leaves and Petals. 17 
venous colours, like red cabbage, &c. All dark shades of green are 
the effect of an immense crowding together of green cells in the 
rete, as may be readily seen in the yew, the bay, the holly, &c. ; 
and all those lesser variations and shades, such as brownish and 
reddish, and a number of others, which add so much to the 
beauty of each leaf and to the picturesque effect of the whole, are 
due (with the above exceptions) to the different colours of the 
cells in the rete. Of this kind are the shadmgs in the leaf of the 
common wild ivy ; the reddish tips and edges of rose- and peony- 
leaves, the purple of the cornel and cineraria, and many similar 
appearances. 
The colowrs of the substance, on the contrary, are marked by 
their want of intensity and by their extreme simplicity. Few 
coloured cells are found in this structure. In petals it is either 
white or lightish, or some faint shade of the general colour of the 
flower. It requires some care to show this m small specimens ; 
but in large ones, such as garden poppies or peonies, the enticle 
and rete can be easily peeled off on both sides, and the colourless 
substance shown in a distinct form, having the exact shape of 
the petal. 
The substance in leaves is always green, except in the light 
parts of variegated leaves, or in leaves of unusual thickness, like 
the aloe. With these exceptions, there is but little difference in 
the shade of green between the substance of one leaf and that of 
another, taking them, of course, in a state of health and matu- 
rity. Thus, in the holly and ivy, the substance is very little 
darker than it is in the beech or laurel. In a great number of 
leaves the difference of shade is not perceptible ; and even in the 
holm-oak, remarkable for its gloomy foliage, the green of the 
substance is not by any means so dark as might be imagined. 
Tt will thus be seen that the coloured cells both in leaves and 
petals are chiefly placed in the rete. A few are occasionally 
found in the substance of petals, and a certain number in that of 
leaves ; but not in general sufficient to determine the outward 
colour. 
2. The size, form and number of the coloured cells always vary 
with the intensity of the general colour of the structure. When 
the colour is very deep the cells are small, roundish, and densely 
packed together in immense numbers. This is their appearance 
in the rete. If the colour is lighter the cells are larger, more 
elongated, and less closely packed together, as they are seen in 
the substance of leaves, and of those petals which are somewhat 
coloured throughout ; and where there is little or no colour, as 
in the substance of the greater number of petals, the cells are 
generally large and oblong, often muriform, and with distinct 
intercellular passages. 
Ann. & Mag. N. Hist. Vol. xvi. C 
