HORSETAILS AND CLUB-MOSSES 205 
posed. In this exposed part archegonia appear, and 
therefore the large spore produces a female gametophyte 
(Fig. 197). 
In Selaginelia, 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 Fie. 197.—Female gametophyte of Selaginella, 
spore (megas pore) pro- having burst through the wall of the mega- 
a spore (m), and bearing archegonia (a) and 
duces the female ga rhizoids (r) upon its exposed part; somewhat 
metophyte. It must be diagrammatic. 
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—=°>o0—S—o—G_? > o—S—o—G_> o—8, ete. 
Selaginella: @—~°> 0—S—°—8°> 089 E> 0, ete. 
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 
