31 
and every other pair stands in the same relative 
position. From a variety of disturbing causes, how¬ 
ever, this fact is overlooked, but it may be generally 
observed; and this crossing, so apparent in some 
kinds of plants, is only a representation of the typical 
manner in which all leaves grow upon a stem. In 
such a plant as the decussate Pimelea, we have this 
mode of formation shown very plainly, in consequence 
of the leaves of that plant being stiff, without flexibi¬ 
lity and without stalks, so that they cannot be bent 
out of their places. 
70. The mere forms of leaves are infinite, and 
scarcely require to be dwelt upon, because they readily 
catch the eye, and are generally tolerably well repre¬ 
sented. To one or two circumstances, however, it is 
material to draw attention. One is the manner in which 
the edge of a leaf is indented or notched. We all 
know that leaves are indented in various ways , but it 
is not a matter of indifference whether they are repre¬ 
sented with one form of indentation or another ; the 
truth being, that their edge’is always of some peculiar 
figure in the same species. In one the leaf has 
notches all round it, appearing as if intended origi¬ 
nally to be sharp-pointed, like a saw, but afterwards 
had the points rounded off. In another plant, each 
of the principal notches is itself notched in a similar 
manner, on a smaller scale. Here, again, we see how 
carefully symmetry is preserved; first we have the 
general form in itself symmetrical, then each portion 
is subdivided in the same symmetrical way, from one 
end to the other ; so that, even in so small a matter as 
the indentations of a leaf, regularity of structure is 
perfectly maintained. 
71. But it is more especially in the veins of leaves 
B 4 
