GENERAL MORPHOLOGY 299 



Phlegmaria, showing a terminal strebilus, with sporangia, and the sporophylls 

 smaller than those borne by the more lax region of the shoot below (Fig. 

 147). The leaves are arranged in whorls a condition not unknown among 

 species of the Phlegmaria group. The sporangia do not appear to have been 

 restricted to the terminal strobilus, but to have occurred also in relation 

 to the larger foliage leaves : this is a condition which has been seen 

 to occur in Z. varium, as well as in the living species of the group 

 sub-Selago, from which the Phlegmaria group appear to be a specialised 

 offset. So far from this distribution of the sporangia raising a difficulty, 

 it seems to point to the existence in very early strata of a Lycopodinous 

 type showing characters which exist in living species, and which com- 

 parison indicates as primitive. These fossils are unfortunately rare, and 

 in the particular case of L. Stockii the essential facts are based upon a 

 single specimen. 



B. LIGULATAE. 



The ligulate Lycopodiales resemble the Eligulatae in general habit, 

 but they differ from them in the presence of a small process the ligule 

 borne on the upper surface of the leaf, near its base : also whereas the 

 living Eligulatae are all homosporous, all the living Ligulatae are hetero- 

 sporous. Selaginella is the preponderant genus of the living Ligulatae : 

 its vegetative development is characterised by frequently repeated branch- 

 ing of the axis, which bears numerous small leaves : but whereas in 

 Lycopodium the dorsiventral development of the shoot is the exception, 

 and the radial the rule, in Selaginella only a few species show the radial 

 construction as a permanent character : the latter, as Goebel remarks, 1 

 usually grow on dryer and brighter spots than the dorsiventral. As 

 the result of experiments on species such as S. sanguinolenta^ in which 

 anisophylly is not constant, but appears under the influence of external 

 factors, Goebel concludes that the dorsiventrality is a phenomenon of 

 adaptation brought about by light : thus the radial type will naturally 

 be the more primitive. In the great majority of the living species, 

 however, the strobilus is isophyllous, even where the vegetative shoot 

 is anisophyllous : thus indicating that it is the more conservative part 

 of the plants. But in ' some ten per cent, of the living species the 

 strobilus itself is also anisophyllous. The definition of the strobilus from 

 the vegetative shoot in Selaginella is more marked than in Lycopodium : 

 a condition corresponding to that of the Se/ag0-group of Lycopodium is 

 unknown, nor have isolated sporangia ever been observed in the vegeta- 

 tive region : the differentiation of the sporophyte of the genus as a whole 

 corresponds to that of the more specialised types of Lycopodium. But 

 imperfect sporangia have been observed at the base of the strobilus of 

 .S. spinulosa, and Martensii, and would doubtless be found in many 

 other species : this condition is open to the same interpretation as 



1 Organography ', vol. i., p. 105. 



