Mr. E. C. Noiu'se 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 shadings 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 colours 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 in small specimens ; 

 but in large ones, such as garden poppies or peonies, the cuticle 

 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. 



It 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. ^ May. N. Hist. VoLxwi. C 



