ALTERNATION OF GENERATIONS. 



47r 



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 archegoma. 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, have 

 been seen. Included in the Lycopodinse is the genus SelagineUa (cf. fig. Ill \ 

 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 maorospores and 



Fig. 360.— Alternatioi. of Generations in Mosses. 

 1 A spore germinating. '■^ A moss-protonema. ^ Protonema giving rise to a bud from wliicli will arise a leafy moss-slioot. 

 4 Longitudinal section of tip of a male slioot of a moss-plant; antlieridia are present between the scales. » Tip Of a female- 

 shoot with archegouia; 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. 3 X 350-400; * X 15; 5 x SO; « X 6. 



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

 macrospores 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 maerospore 

 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 archegonium 

 gives rise to a new, asexual SelagineUa plant. 



This differentiation amongst the spores in SelagineUa (in consequence of whicL 

 the plant is termed heterosporous, in contradistinction to Ferns, and Lycopodium,. 



