494 
NATURE 
| March 22, 1883 
may be brought about. Figs. 31, 32, and 33 exhibit a 
slightly different form of the same tendency. 
That this is the real origin of pinnate venation seems 
pretty clear on a comparison of a good many otherwise 
closely related forms. Look for example first at the rounded, 
almost orbicular leaf of Geranium molle and its allies, 
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Fic. 23.—Development of Dicotyledonous seedling. 
\ 
with palmate ribs ; and then look at the long, narrower, 
doubly pinnate, and pinnately-ribbed leaves of Erodium 
cicutartum. Or again, look at the common cinquefoil, 
erect and palmate; and then at silver-weed, long, creep- 
ing, closely pressed to the ground, and with numerous 
pinnate leaflets. Once more, compare A/chemil/a with 
Fic. 24.—Acacia melanoxylon. 
Poterium and Sanguisorba. As a still simpler instance, 
where we get the difference in its first beginning, contrast 
Ranunculus acris with R. repens, or the least compound 
leaves of the blackberry bramble with its own most com- 
pound foliage. Asa rule the most pinnate groups, such 
Fic. 25.—Typical Monocotyledonous leaves and venation. 
as the lesser crucifers, the peaflowers, &c., have very 
long leaves. 
This suggested origin of pinnate venation in dicotyle- 
dons becomes even more probable when we look at the 
pinnate members of other classes. Among monocotyle- 
dons the long-leaved arums, though their venation is 
fundamentally parallel in type, have yet acquired a 
branching and practically pinnate set of ribs. The 
plantains and bananas, with very long and broad foliage, 
carry the same tendency yet further; for their leaves are 
pinnately ribbed from a stout midrib. The lower shrubby 
or bushy palms, like Chamzrops, have fan-shaped leaves, 
with veins diverging in rough parallelism from a common 
centre ; that is to say, they are in fact palmate; but in 
the taller arborescent palms, with their long leaves, the 
internodes of the midrib (to use the same convenient 
Fic. 26.—Typical Dicotyledonous leaf and venation. 
phrase once more) are fully developed, so that the leaf 
becomes pinnatifid. In this case the subdivision into 
leaflets is probably protective against tropical storms. 
The broad-leaved plantains and the Chameerops, though 
so much shorter than the pinnate palms, are often torn 
by the wind, and a plantain leaf so torn into ribbons 
| closely resembles a cocoanut leaf; in the taller palms 
this disruption between the ribs becomes norma]. Com- 
pare Zamia and the other cycads among gymnosperms. 
Fic. 27.—Sazitttaria sagittifolia. 
Once more, the ferns are a class with long lanceolate 
fronds as a rule, and their venation is almost always 
pinnate ; the only ferns that vary much from the central 
type being some like the Maidenhairs, which are tufty 
and rather ovate in general form, and have so modified 
their venation as closely to approach the herb Roberts 
and other hedgerow plants in the outer effect. We may 
