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" mechanical or structural " conditions on which the colours of leaves 

 and petals are in part dependent. These are, he writes, " 1st, the 

 situation of the coloured cells ; 2nd, their size, form and number; 3rd, 

 their intermixture with each other, and 4th, their degree of visibility. 

 He distinguished the parts of a leaf or petal into three, namely, " the 

 substance^ consisting of cellular tissue and venous ramifications, the 

 cuticle, or epidermis, and a layer of cells immediately beneath the cu- 

 ticle, to which we may give the provisional name of retey Mr. 

 Nourse states that " the situation of the coloured cells is different in 

 leaves and petals, though their general structure is anatomically simi- 

 lar," He adds that in petals the rate " 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 sim- 

 ply stripping it off with the cuticle." He also considers that "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 vinous colours, like red cabbage, &c." But he 

 says that the green colour of leaves is itself in the substance of the 

 leaf, while in petals the substance is " either white or lightish, or some 

 faint shade of the general colour of the flower." We suspect — nay, 

 we are assured by individual observation — that there is some error 

 in the observations of Mr. Nourse, or, rather, he may have observed 

 correctly, and yet may have compared incorrectly. It would lead us 

 into an incompatible length of detail, were we to go fully into this 

 question ; but a single fact will probably suffice as a hint to Mr. 

 Nourse himself. There is a pretty dwarf tulip often brought into 

 London in flower-pots, between Christmas and spring, and familiarly 

 known to gardeners under the name of the ' Duke Van Thol.' Its 

 colour is orange-red, with a bright yellow margin. If Mr. Nourse 

 will strip off the cuticle from both sides of a petal, he will remove the 

 whole of the red colour, through the adhesion of the rete to the cuti- 

 cle. The whole substance of the leaf will then be found of a bright 

 yellow colour, similar to the yellow margins of the entire petal. It is 

 true the colour of the margin of the entire petal is somewhat brighter, 

 but this appears to be only because part of the yellow colouring mat- 

 ter of the margin is situate in the rete, and the greater thickness of 

 colour (in the rete and substance together) is seen through a transpa- 

 rent and shining membrane, namely, the cuticle. We may safely as- 

 sert, with this easily-reached fact before us, that the general law of 

 position of colour is the same for leaves and petals. Doubtless there 

 are flowers in which the interior substance of the petals is pale, but 



