630 GENERAL COMPARISON OF THE FILICALES 
leaf and axis as branches of a dichotomy. The leaves always originate 
monopodially. Secondly, other Pteridophytes, such as the Sphenophylls 
and early Calamarians, exist with bifurcate leaves, but without any 
suggestion of an origin of axis and leaf from a common dichotomous 
system. These grounds, over and above the inherent improbability of 
the comparisons with Fucoids introduced by Potonié, or with the game- 
tophyte of Liverworts by other writers, should suffice to show that the 
suggestion which F threw out in 1884 is untenable, as I very soon realised 
it to be. All developmental evidence shows that the axis in Ferns, as 
in other plants, was throughout descent a part of distinct origin from 
the leaves which it bears. 
The dichotomous theory of origin of the whole shoot, including axis 
and leaf, has been supported also by Tansley on the basis of anatomy ; and ° 
it has been pointed out that analogies exist between the structure of axis 
and of leaf in certain early fossils... Especially it has been shown that there 
is an approach to a radial type of construction of the lower region of the 
leaf in ‘certain cases. It need be no surprise that such similarities to 
the structure of the axis should exist in an appendage which is a part 
of the same shoot as the axis; as the leaf became larger and more 
important its requirements would become similar to those of an axis: to 
meet these a structure analogous to that of the stem would then be 
probable, such as is actually seen. In the facts adduced I see nothing 
stronger than structural analogies: this class of evidence carries little 
weight as against the objective fact that in living Ferns the leaf is always 
seen to arise monopodially. Thus the dichotomous theory, which is based 
on analogies, appears to break down in the absence of developmental 
fact. 
It is possible now to institute a comparison of the shoot of Ferns with 
that of other Pteridophytes, and to consider its relation to the theory of the 
strobilus. In its original radial structure, with derivative dorsiventrality, 
and in its occasional dichotomous branching it corresponds to other 
strobiloid types. The genetic relation of leaf to axis as actually observed 
is the same, and in point of fact it is in the proportion of leaf to axis 
and in the architecture of the leaf that the chief difference lies. But among 
strobiloid types, and especially among their fossil representatives, the leaf 
is not always small or simple: the leaves of certain living Lycopods 
(Z. serratum and Jsoetes) are relatively large, as were also those of some 
of the fossils, notably Sigi//aria. The branched leaves of the Spheno- 
phylleae and Psilotaceae, and even of some of the Calamarians, such as 
Archaeocalamites, and notably of Pseudobornia, are instances of branching 
of leaves in strobiloid forms. Again, in our view a great leaf-enlargement 
in a fundamentally strobiloid type has resulted in the Ophioglossaceae. Thus 
variety in size and complexity of the leaves existed in other Pteridophytes 
besides the Ferns. Even the dichotomy which is so frequent in the first 
1New Phytologist, 1907. 
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