EMBRYOGENESIS IN THE LYCOPODINEAE 103 



occurrence in the ancestry of vascular plants. This view did not receive 

 the general approval of Treub's contemporaries and is now little more 

 than an interesting idea. To Bower (1907, 1935), the protocorm was 

 not so much a primitive organ as an 'opportunist growth' ; and even 

 though it may be of ecological importance in establishing the young 

 sporophyte of certain species, it should not be regarded as a phylo- 

 genetic feature in the Lycopodiaceae, still less in vascular plants as a 

 whole. Browne (1913), impressed by the great stem development in 

 Palaeozoic lycopods, suggested that the protocorm might be a modified 

 and reduced stem. The adaptive value of the embryonic swellings in 

 lycopods has been freely invoked. 



Bower (1922, 1935) has pointed out that several species of Lycopo- 

 dium, in different ways and in different degrees, tend to a 'parenchy- 

 matisation' during their embryonic development. L. selago has a 

 comparatively small foot and an embryonic growth that might be 

 described as exemplifying a 'normal' axial development; in L. 

 phlegmaria, the parenchymatous foot is more conspicuous; in L. 

 clavatum and L. annotiuum there is a considerably enlarged parenchy- 

 matous foot; while in L. cernuum and L. laterale a parenchymatous 

 foot and a greatly enlarged tuberous shoot or protocorm are formed. 



Bower (1935) has related the differences between the selago and 

 clavatum types of embryo to the prothalli which bear them. The 

 prothallus of L. selago may develop at or near the surface, it may be 

 partially green and autotrophic and partially mycorrhizic, and the 

 nursing period of the embryo may be relatively brief. In L. clavatum, 

 on the other hand, the prothallus is a large, cakelike, underground 

 structure, depending entirely on mycorrhizal nutrition. The formation 

 of a large swollen foot has been described by Bower as an 'intra- 

 prothallial haustorium,' and as 'an opportunist growth.' These details 

 are considered by him to accord with the view that 'the embryo is long 

 dependent for nourishment entirely upon the large prothallus; hence 

 its swollen haustorial foot, which is developed most strongly in the 

 direction of the largest nutritive supply.' 



Position and Nutrition of the Embryo. All lycopod embryos are 

 initially nourished by the prothallus. In some species the embryo may 

 be completely immersed in the prothallial tissue for a considerable 

 time. Bower (1908, 1922, 1935) has given some attention to relevant 

 nutritional aspects. In his view the endoscopic embryogeny of Lycopo- 

 diunu with its consequential deep immersion of the embryo in the 

 prothallial tissue, has 'doubtless an immediate advantage in point of 

 nutrition of the embryo in its earliest stage. But the ultimate end is the 

 establishment of a young plant with an upward-growing shoot suitable 

 for self-nutrition. The orientation of the archegonium will then be a 



