16 Mr. E. C. Nourse on the Colours of Leaves and Petals. 



IV. — On the Colours of Leaves and Petals. By William 

 E. C. Nourse, M.E.C.S. 



The colours of leaves and petals depend on several conditions ; 

 some mechanical or structural, and some chemical. The latter 

 have been made the subject of many iiivestigations. The former, 

 though requiring little more than common observation, have 

 been passed ovtjr, or but slightly noticed. It is to a clearer 

 knowledge of these that the present paper, so far as it goes, is 

 intended to contribute. 



The structural or mechanical circumstances which influence 

 the colours are, 1st, the situation of the coloured cells ; 2nd, their 

 size, form and number ; 3rd, their mixture with each other ; and 

 4th, their degree of visibility. 



1. The situation of the coloured cells is diiOferent in leaves and 

 petals, though their general structure is anatomically similar. If 

 a leaf be torn, the green colour appears to be in the central sub- 

 stance ; while in a petal the centre is nearly white, and the co- 

 lour peels off with the cuticle. This difference merits a degree 

 of attention which has not hitherto been paid to it. 



The structural parts of a leaf or petal are, the substance, con- 

 sisting of cellular tissue and venous ramifications ; the cuticle, or 

 epidermis ; and a layer of cells immediately beneath the cuticle, 

 to which we may give the provisional name of Rete. 



This latter structure is seldom mechanically distinct, but is 

 either continuous with the substance, as in leaves, or adherent 

 to the cuticle, as in petals ; though it can sometimes be shown in 

 a separate form in petals of a large size. Its characters, however, 

 effectually distinguish it from the other structures. It is the 

 densest parenchyma of the organ, consisting of an immense 

 number of nearly circular cells without any interstices. But the 

 circumstance which makes it most conspicuous is, that it is the 

 seat of colouring matters which are scarcely found in the other 

 structures. The rete thus appears entitled to be considered as a 

 distinct tissue, and may be found to perform important func- 

 tions, especially in the petals, in which its development is most 

 distinct. 



The colours of the rete possess an almost endless variety, and, 

 in fact, it is in this structure that the most highly coloured cells 

 are always found. Of petals, it contains the entire colouring ; the 

 yellow, red, blue, brown, black, and all the intermediate tints are 

 wholly produced in its cells, and can be completely removed by 

 simply stripping it off with the cuticle. This can be easily done 

 with any common flower. In leaves the rete is the seat of all the 

 modifications of the green colour which those organs present, 

 excepting variegation, cuticular changes, and what may be called 



