SUMMARY 365 



assimilating leaves. In other cases it shows various modifications. Where 

 the thallus is buried deeply underground, as in L. 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 iw;//////-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, Selaginella spinulosa, 

 which is held as a relatively primitive type on comparison of its mature 

 sporophyte, the seedling is very similar to that of Z. Selago, 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 ernbryogeny of Isoetes 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 Selaginella. 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 

 :onsiderable 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 ot 

 minute investigation throughout their life-cycle : and, thirdly, because these 

 and the fossils together how 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. 



