SUMMARY 365 
assimilating leaves. In other cases it shows various modifications. Where 
the thallus is buried deeply underground, as in Z. clavatum, the lower tier 
of the embryo enlarges as an haustorial foot, while the first leaves are 
modified into colourless protective scales, evidently a secondary condition. 
In the cernuum-type and in Phylloglossum a distinct extra-prothallial 
swelling appears in the upper tier of the embryo, disturbing the position 
and even the arrangement of its parts. Since the first stages of this 
embryo resemble those of other Lycopods, and since the normal shoot, when 
ultimately established, is also of the usual Lycopod type, it is concluded 
that the swollen stage, styled the “protocorm” by Treub, is a gouty 
interlude, introduced secondarily into the normal development, and not a 
stage of general significance. In the ligulate series, Se/aginella spinulosa, 
which is held as a relatively primitive type on comparison of its mature 
sporophyte, the seedling is very similar to that of Z. Se/ago, notwithstanding 
the striking difference of their prothalli. But the simple spindle-form 
which it shows is departed from in other species, by the lateral formation 
of a swollen haustorium: this “foot” is again held to be a secondary 
development. The apparently divergent embryogeny of Jsvefes is carried 
out without a suspensor, but the position of the parts in relation to the 
greatly abbreviated axis is essentially similar to that in Sedagine/la. It thus 
appears that in both series the most primitive type has an embryo in 
the form of a simple spindle: it forms its first leaves as normal green 
foliage leaves, and those species in which this is departed from are held 
as the result of secondary modification. The first foliage leaves in these 
simple forms differ in no essential respect from the subsequently formed 
sporophylls, except in the absence of the sporangium. Hence the observed 
facts support the view that all the leaves were originally sporophylls, and 
the whole plant originally a simple strobilus. 
It has thus been seen that a strobiloid theory is applicable to all 
‘known types of the Lycopodiales. This matter has been dealt with at 
considerable length because, in the first place, this phylum of Vascular 
Plants dates back fully as far as any other in the Palaeontological record. 
Secondly, because it is represented by many living species susceptible of 
minute investigation throughout their life-cycle: and, thirdly, because these 
and the fossils together show gradual, and at the same time considerable 
divergence of detail in the one uniform scheme. They thus provide a 
better basis for comparison than any other series of Pteridophytes of equal 
age. The conclusions arrived at will be susceptible of comparison with 
those relating to other phyla of Vascular Plants. But though the applica- 
tion of the theory of the strobilus may be extended to other phyla, it 
must be remembered that the arguments and conclusions relative to the 
Lycopodiales stand by themselves, and would still be equally cogent if no 
other Vascular Plants existed on the earth’s surface. 
