HORSETAILS AND CLUB-MOSSES 



205 



posed. In this exposed part archegonia appear, and 

 therefore the large spore produces a female gametophyte 

 (Fig. 197). 



In Selaginella, there- 

 fore, the two kinds of 

 sex-organs are produced 

 by different plants, and 

 we speak of male and 

 female gametophytes. 

 The connection of these 

 two kinds of gameto- 

 phytes with the two 

 kinds of spores must be 

 kept clear. The small 

 spore (microspore) pro- 

 duces the male gameto- 

 phyte, and the large 

 spore (megaspore) pro- 

 duces the female ga- 

 metophyte. It must be 

 remembered, also, that 



with this change the gametophytes have become much 

 smaller than they were before, and are no longer indepen- 

 dent, in the sense of doing chlorophyll work. 



It follows that in the life-history of Selaginella there is 

 an alternation of the sporophyte with two gametophytes. 

 How this contrasts with the life-history of an ordinary fern 

 may be indicated as follows: 



Fern: G~2>o S o G~o>o S o Gn2>o S, etc. 

 Selaginella: gn2>o SzzZ~2> o SnZ >> etc - 



119. Coal. The ferns, equisetums, and lycopods were 

 associated together during the Coal-measures, and were 

 the most conspicuous plants in the formation of coal. The 

 formation of peat, already referred to ( 101), indicates the 



FIG. 197. Female gametophyte of Selaginella, 

 having burst through the wall of the mega- 

 spore (m), and bearing archegonia (a) and 

 rhizoids (r) upon its exposed part; somewhat 

 diagrammatic. 



