18 Mr.E.C. Nourse on the Colours of Leaves and Petals. 
In white flowers, the cells which contain opake white matter 
are always rounder and more thickly packed together than the 
empty cells. 
3. Tints may be produced by the mere mechanical mixture of 
the coloured cells. In these cases no union of the colours takes 
place, but they remain distinct in their separate cells, side by side. 
When the cells are mixed with regularity, a uniform tint results ; 
but when the colours are more or less massed together, variega- 
tion or marking is the consequence. Coloured cells sometimes 
lie over one another, causing a new tint by one layer being seen 
through the other. 
The leaf of the Pelargonium zonale is well-known for its pecu- 
liar dark stam. This is entirely in the rete, for the substance of 
the leaf is pure green. The rete however, viewed carefully in 
various sections beneath the microscope, appears to consist, not 
of dark cells, but of distinct red and green ones, very minute, 
densely packed together and intermingled ; and it is by this juxta- 
position of the red and green, and by the green cells of the sub- 
stance being partially visible through it, that the effect of a dark 
tint seems to be caused. The leaf of the variegated elder pre- 
sents an appearance of similar origin. Some parts of the leaf are 
of a decided green, and some almost white; but there are also 
patches of a sort of imperfect green, paler, and somewhat glau- 
cous. The substance in these parts is not less green than in the 
darkest parts of the leaf, as may be seen by looking at the under 
surface ; but the rete, instead of containmg dark green cells, con- 
sists of a thin layer of white ones; and these, with the cuticle, 
to which they are adherent, by lymg over the green substance, 
produce the glaucous appearance. 
4. The cuticle in this instance contributes to the effect. This 
structure has not yet been mentioned, because coloured cells are 
never found in it; and it merely modifies the appearances of co- 
lows by regulating the visibility of the coloured cells. This is 
so obvious, that it only needs to be referred to, as well as the 
effects of the cuticular appendages. 
In most petals the cuticle is extremely delicate ; often consist- 
ing of the finest web, impossible to be detached, and only to be 
seen occasionally at the carefully torn edges of a flower. It js 
somewhat thicker in large petals, and can then be raised and torn 
off in shreds. Cf course, in these instances, it is perfectly trans- 
parent, and permits the colours to be seen through it in the most 
distinct manner. 
Such are the structural circumstances reiating to the colours 
of leaves and petals. Simple as they are, and easily observed, 
they required to be stated, to receive their proper share of atten- 
tion. The chief pomts about them are, the anatomical differences 
