March 8, 1883 | 
NATURE 
441 
it modifies that type so far as possible to suit the exigen- 
cies of its altered conditions. It cannot remake the leaf 
de novo at each change of habit or habitat : it can only 
remodel it in accordance with certain relatively fixed 
ancestral patterns. Hence, as a rule, each great group 
of plants—family, tribe, or genus—has a common type of 
leaf to which all its members more or less closely ap- 
proximate. Occasionally, as among the composites, the 
diversity of types in a single family is very great; at 
other times, as among the peas and still more among the 
pinks, the type is fairly well preserved throughout. But, 
Fic. 5. 
in spite of all apparent exceptions, and of numerous very 
divergent cases, there is a general tendency in most 
allied plants to conform more or less markedly to a cer- 
tain general central and ideal form of leaf—the form from 
which all alike are hereditarily descended with various 
modifications. The actual shape in each case is not the 
ideally-best shape for the particular conditions ; it is only 
the best possible adaptive modification of a pre-existing 
hereditary type. 
_ The point that is most common to leaves of different 
Fic. 6. 
sorts in the same group is their vascular framework or 
ground-plan ; in other words, their venation. This is the 
typical thing which tends most of all to reproduce itself, 
under all varieties of external configuration. The plant 
seems to build up first, as it were, its ancestral skeleton, 
and then, if it can afford material, to flesh it out with the 
intervening cellular tissue (not, of course, literally, for all 
the leaf buds out at once froma single knob). A glance 
at the accompanying diagrams will show how easily, by 
failure of growth in the intervals between the principal 
ribs, a simple primitive rounded leaf may be converted 
during the course of evolution into a lobed or compound 
one. In Fig. 1 we have such an ovate leaf, with digitate 
venation: the dotted line marks the chief intervals 
between the ribs, mainly filled by cellular tissue. In 
Fig. 2 we have the leaf of a sycamore, with the same 
venation, but with the intervals between the ribs unfilled. 
Here it will be noticed that the apex of the five main 
lobes corresponds in each case with the termination of a 
main rib ; and the largest lobe answers to the midrib. Simi- 
larly, the apex of each minor serration answers to the 
termination of a secondary riblet. The type remains the 
KAO 
: /) 
Fic. 7. 
same throughout; only in Fig. 1, material has been sup- 
plied to fill it all in, and in Fig. 2, only enough has been 
supplied to cover the immediate neighbourhood of the 
main veins. 
In Figs. 3 and 4 we get a further modification of a 
similar type. Here the cutting of the lobes goes so deep 
as to divide the entire blade into separate leaflets ; and 
the result is the compound leaf of the horse-chestnut. 
The same thing may also occur with pinnately-veined 
leaves. In Fig. 5 we get a typical leaf of this character, 
where the supply of carbonic acid and sunshine under the 
Fic. 8. 
Fis. 9. 
average circumstances of the plant is sufficient to allow 
of its having assumed a full and rounded specific form. 
Fig. 6 shows the less fully-veined tracts in such a type of 
foliage ; and in Fig. 7, where the ordinary conditions do 
not favour full development, we get the familiar irregu- 
larly-lobed blade of the English oak. The diagrammatic 
representation in Fig. 8 suggests the steps by which a 
regularly pinnately-veined leaf, such as that of the common 
olive, may pass into a pinnatified and pinnatisect form 
by non-development of the mainly cellular tracts. We 
may thus get either a lobed leaf like the hawthorn, as 
