ALTERNATION OF GENERATIONS. 



477 



in cones of closely-fitting scales, each of which bears a sporangium. The spores in 

 Lycopodium are all alike, and on germination form prothallia, which bear antheridia 

 and archegonia. It is interesting to note in passing that we have only become 

 acquainted with these prothallia in recent years, and for the most part in exotic 

 species. The prothallia of a limited number of European species, however, liave 

 been seen. Included in the Lycopodinae is the genus Selaginella (cf. fig. IIP, 

 vol. i. p. 421), resembling Lycopodium in its moss-like habit, but differing from it 

 in that two sorts of spores are produced. These spores, known as maerospores and 



Fig. 350.— AlternatioL of Generations in Mosses. 

 1 A spore germinating. 2 a nioss-protonema. ^ Protonema giving rise to a bud from which will arise a leafy moss-shoot. 

 * Longitudinal section of tip of a male shoot of a moss-plant; antheridia are present between the scales. » Tip of a female 

 shoot with archegonia; two of them have much enlarged due to the swelling of the young sporogoniums within. « Leafy 

 female shoot of a moss-plant with fully developed sporogonium at its tip. Spores arise asexually in the terminal capsule. 

 1. 2, s X 350-400 ; * X 15 ; 6 X 80 ; 6 X 5. 



microspores, arise in different sporangia in the same cone, in many cases. The 

 maerospores are relatively large, and are contained four in a sporangium; the 

 microspores are small, and a large number of them is present in a sporangium. 

 The prothallia resulting from their germination are of two kinds; the macrospore 

 gives rise to a female prothallium which bears archegonia; the microspore to a 

 much reduced male prothallium bearing a single antheridium. The sexual genera- 

 tion — which in the Fern consists of one structure, the prothallium — here consists of 

 two structures, the male and female prothallia. After fertilization the arcliegonium 

 gives rise to a new, asexual Selaginella plant. 



This differentiation amongst the spores in Selaginella (in consequence of which 

 the plant is termed heterosporous, in contradistinction to Fei-ns, and Lycopodium, 



