LEAF IN THE VASCULAR CRYPTOGAMS AND GYMNOSPERMS 
585 
uniform : speaking in a general sense, it is a winged structure, the wings being a 
subsequent development upon the more bulky central column (compare Osmunda). 
In connexion with the further development of the leaf, and the formation of the 
pinnae, there are two questions to be decided : First, whether the pinnae are formed 
monopodially or by dichotomy of the apex of the phyllopodium ; and secondly, if 
they are formed monopodially, what is the order of their development ? 
In connexion with the former of these questions, it may be called to mind that a 
sympodial development of the leaf in the Cycadacece has been suggested by Sachs 
(Text Book, 2nd Engl. Edn., p. 503), on the ground of the conformation of the apex 
of the mature leaf: this idea is still maintained by some (Fankiiauser ; Ginkgo biloba 
p. 8, Bern, 1882), though the observations of Warming (‘ Recherches et remarques sur 
les Cycadees,’ Copenhague, 1877), which it must be confessed are not very complete, 
point to a monopodial development. My own observations show that in Cycas See- 
manni (as also in all the Cycadacece in which I have had the opportunity of investi¬ 
gating this point) the development is monopodial. It is on the wings or ridges above 
described that pinnae first make their appearance, and clearly below the extreme apex 
of the phyllopodium, which takes no direct part in their formation. There is first 
seen a slight undulation of the surface of the wings : this becomes gradually more 
pronounced, while the convexities gradually round themselves off, and finally appear 
as smooth hemispherical swellings. Still, though some of the pinnae are formed by 
monopodial branching, there might be a gradual transition to a sympodial development 
of those formed later : a transition which probably does take place in many Ferns. 
This is naturally out of the question in those of the Cycadacece, to be described below, 
in which the order of succession of the pinnae is exclusively basipetal. As to the 
remaining forms {Cycas, Dioon), the branching seems always to be monopodial, since 
the apex of the phyllopodium is throughout a much more bulky mass of tissue than 
the young pinnae formed on it. It may therefore be concluded that the sympodial 
appearance of the apex of the mature leaf is misleading. 
In approaching the second question, as to the order of succession of appearance of the 
pinnae, it is obvious that their earliest stages of development should be observed ; it 
is not sufficient to note the relative size of pinnae, which have already passed the 
earliest stages ; though such observations will give useful evidence, still they cannot be 
regarded as conclusive. In illustration of this I may quote an example, represented 
in Plate 38, fig. 26. In this case, passing from below upwards, pinnae of various sizes 
are encountered, yet I have never had any direct evidence of irregularity in succession 
of formation of the pinnae in any of the Cycadacece. Again, certain pinnae may be 
arrested in their development at a comparatively early age (Plate 39, fig. 31) ; thus 
arguments drawn from any but the very earliest stages of development, though they 
may be admitted as secondary evidence, cannot be adopted for the ultimate decision 
of the question. Now, to judge from Warming’s drawings ( l.c ., Taf. 3, figs. 19, 20), 
and from the description given by Karsten (‘ Organographische Betraclitung der Zamia 
MDCCCLXXXIV. 4 F 
