492 MOSSES AND FERNS chap. 



form successively from a group of meristematic cells, which 

 usually develop into the permanent apex of the stem. About 

 the time that the stem apex becomes recognisable as such, the 

 first root appears as a surface outgrowth of the protocorm, 

 and strictly exogenous in origin. Not infrequently the end 

 of the primary root gives rise to a tubercle similar to the proto- 

 corm. 



An interesting case was seen by Treub, where, apparently 

 by a longitudinal division of the young embryo, two embryos 

 were formed, much as is normally the case in some Gymno- 

 sperms. 



On comparing the two types of embryo found in L. phleg- 

 maria and L. cermmm, the main differences are the almost 

 complete absence of the protocorm and greater development of 

 the suspensor in the former. L. inundatum, as might be ex- 

 pected, corresponds closely in the structure of the young sporo- 

 phyte to L. cernimm. 



Corresponding with the late appearance of the roots is the 

 late development of the vascular bundles, which, according to 

 Treub, are often quite absent from the cotyledon and even 

 occasionally from the second leaf. The protocorm of L. cer- 

 nuum and L. inundatum Treub regards as the remains of a 

 primitive structure originally possessed by the Pteridophytes, 

 which replaced the definite leafy axis found in the more special- 

 ised existing forms. 



Phylloglossum, which has sometimes been regarded as the 

 most primitive of existing Pteridophytes, resembles closely the 

 young sporophyte of Lye op odium cernuum. 



Bruchmann states ((4), p. 38) that the fertilised egg en- 

 larges very much before the first division wall is formed, differ- 

 ing in this respect from Selaginella, and more resembling Ma- 

 rattia or Botrychium. The first division is transverse. The 

 larger of the two cells, lying next the archegonium-neck, forms 

 the suspensor, and the smaller one develops into the embryo 

 itself. 



Both L. clavatum and L. annotinum differ from the species 

 studied by Treub in the late development of the leaves (Bruch- 

 mann (4), p. 46). Moreover, in these species there are two 

 opposite cotyledons as in Selaginella. 



The development of the young sporophyte is extraordi- 

 narily slow, and Bruchmann states that it sometimes does not 



