442 
NATURE 
| March 8, 1883 
adumbrated at the summit of the diagram, or a compound 
leaf with pinnate leaflets like the commonest papiliona- 
ceous type, as shown in the lower portion. These 
examples will at once make clear the principle that with 
very slight changes in the real structural composition of 
a leaf we may have very great differences in the resulting 
outline. How the various underlying types of venation 
themselves are acquired or modified we must consider at 
a later stage; for the present we must take them for | 
granted as relatively fixed generic or tribal charac- 
teristics. 
It may be necessary to warn the reader in passing that 
comparatively little importance must be attached to the 
particular circumstances of each individual leaf. It is 
the average circumstances of the species which give rise 
to the specific type. True, each particular blade cannot 
grow at all except in so far as material is supplied to it 
during its growth from the older and more settled 
members of the complex plant-commonwealth ; but even 
when such material is supplied to it, it will only grow to 
the extent and into the shape which natural selection has 
shown to be the best on the average for all its prede- 
cessors. For example, no plethora of available material 
would make the sycamore or the oak produce leaves like 
those represented in Figs. 1 and 5; it would only make 
them produce a greater number of normal leaves like 
those represented in Figs. 2 and 7, since these embody 
the final result of all the past experience of the race—the 
residuum of countless generations of unsparing selection. 
A single illustration of the way in which these general 
principles work can best be found, as a first example, in 
the foliage of the water-crowfoot (Ranunculus aguatilis, 
Fig. 9). This well-known plant, growing as it does in 
streams or pools, has two forms of leaf on the self-same 
branch, strikingly different from one another. The lower 
or submerged leaves, which wave freely to and fro in the 
water, are minutely subdivided into long, almost hair-like, 
filaments ; the upper or floating leaves, which loll upon 
the surface of the stream, are full and rounded, though 
more or less indented at the edge into from three to six 
obovate lobes. Familiar as is this curious little English 
plant, the causes which give it its two types of leaves 
admirably illustrate the laws which we must employ as 
the general key to all the shapes of foliage throughout 
the vegetal kingdom. 
First, as to the submerged leaves. These organs, 
growing in the water under the surface, have not nearly 
so free access to carbonic acid as those which growin the 
open air. For the proportion of carbonic acid held in 
solution by water is very small; and for this small 
amount there is a great competition among the various 
aquatic plants. Asa rule, the cryptogamic flora of fresh 
waters consists of long streaming alge or characex, 
which assume filamentous shapes, and wave about in the 
water so as to catch every passing particle of the precious 
gas. 
take to inhabiting similar spots, their submerged leaves 
also tend to assume somewhat the same forms, and to 
move freely with every current in the pond or stream, so 
as to catch whatever fragments of carbon may happen to 
pass their way. In this case, there is no dearth of sun- 
shine, no interference of other plants with the incidence 
of the light ; the waving thread-like form depends solely | 
upon the comparative want of carbon in the surrounding | 
medium. The leaves have acquired the shape which 
enables them best to lay hold on whatever carbon there 
may be in their neighbourhood ; any other arrangement 
would involve a waste of chlorophyll—a misplacing of 
it in an unadvantageous position. Full round leaves 
would be useless under water, because there would not 
be work enough for them to do there. 
On the other hand, when the leaves reach the surface, 
they have room to spread out unmolested into an area 
singularly free from competing foliage. Here, then, they 
When flowering-plants, like- the water-crowfoot, | 
plim out at once into a larger rounded type, as they can 
obtain abundant carbonic acid from the air around, 
and can catch the unimpeded sunlight on the surface of 
their pond. The two cases, as Lamarck long since re- 
marked, are somewhat analogous to those of gills and 
lungs; for though in the one case it is oxygen that is 
required, and in the other case carbonic acid, yet inas- 
much as both are gases dissolved in water, the parallelism 
on the whole is very close. 
It is to be noted, however, that in both cases the 
central ranunculaceous type of leaf is faithfully preserved 
in the ground-plan or framework. This central type of leat 
is found in a rounded form in the lesser celandine (A. 
| ficaria), and in the radical leaves of the goldilocks (R. 
auricomus). Itis more divided and cut, or (to put the 
same thing conversely) less filled out between the ribs in 
the common meadow buttercup (2. acrzs). But in the 
water-crowfoot, the floating leaves remain very close to the 
rounded form of lesser celandine, though a little more 
lobed at the edge; while in the submerged leaves, we 
get hardly anything more than an attenuated skeleton of 
the venation, still essentially keeping up the typical form, 
though in a somewhat exaggerated and minutely sub- 
divided manner. When one compares these submerged 
Jeaves with the equally filiform and minutely dissected 
submerged foliage of the water-violet (Hottonta palus- 
tris) and the water-milfoil (yriophyllum sficatune), 
one sees at once that the same effect has been obtained 
in the various cases by like modification of wholly unlike 
ancestral forms. While assuming extremely similar 
outer appearances, all these plants retain essentially 
diverse underlying ground-plans. 
Furthermore, there are various minor forms or varieties 
of the water-crowfoot in which minor peculiarities of like 
import may be observed. The form known as &. fluitans 
lives chiefly in rapidly-running streams, where none of its 
leaves can reach the surface; hence all its foliage is 
submerged, and deeply cut into very long, thin, parallel 
segments, which wave up and down in the rapids, and 
are admirably adapted to catch the floating particles of 
carbonic acid carried down by the water in its course. 
The variety known as A. cércinatus grows mainly in deep 
still pools, where also its leaves cannot reach the top; 
and it has likewise submerged foliage with finely-cut 
segments, but the separate pieces are “ shorter and more 
spreading,” because this form is best adapted to catch 
the stray dispersed particles of carbonic acid in the quiet 
waters. . The common type (vz/garis of Bentham) has 
both forms of leaves, floating and submerged, and grows 
mostly in shallow pools or slow streams. The type 
known as ivy-leaved crowfoot (2. hederaceus) creeps on 
mud or ooze, and has only the full three-lobed leaves. 
; Finally, it may be noted that even the particular position 
of individual leaves here counts for scmething; since 
nothing is commoner than to find one of the finely-cut 
submerzed leaves with a few upper segments floating on 
the surface; and these upper segments begin to fill out 
| at once into broader green tips, thus giving the end of 
the leaf an odd, swollen, and bloated appearance. 
GRANT ALLEN 
(To be continued.) 
HERRING AND SALMON FISHERIES 
ANG a meeting of the Executive Committee of the 
Edinburgh International Fisheries Exhibition of 
1882, which proved so successful, held on Wednesday, 
February 28, it was resolved, on the motion of Mr. John 
Murray, seconded by Sheriff Irvine, that the funds at 
the disposal of the Executive Committee be granted to 
the Council of the Scottish Meteorological Society to 
carry out the proposed investigations with reference to the 
herring, salmon, and other fisheries which are described 
