SECTION 17.] 



PTERIDOPHYTES. 



161 



like cavities, and fertilize the cell. This thereupon sets up a growth, forms 

 a vegetable bud, and so develops the new plant. 



491. An essentially similar process of fertilization has been discovered 

 in the preceding and the following families of Pteridophytes ; but it is 

 mostly subterranean and very difficult to observe. 



492. Club-Mosses or Lycopodiums. Some of the common kinds, 

 called Ground Pine, are familiar, being largely used for Christmas wreaths 

 and other decoration. They are low evergreens, some creeping, all with 

 considerable wood in their stems : this thickly beset with small leaves. In 

 the axils of soice of these leaves, or more commonly, in the axils of pecu- 

 liar leaves changed into bracts (as in Fig. 511, 512) spore-cases appear, as 

 roundish or kidney-shaped bodies, of firm texture, opening round the top 

 into two valves, and discharging a great quantity of a very fine yellow 

 powder, the spores. 



493. The Selaginellas have been separated from Lycopodium, which 

 they much resemble, because they produce two kinds of spores, in sepa- 

 rate spore-cases. One kind (Miceoseoues) is just that of Lycopodium ; 

 the other consists of only 

 four large spores (Maceo- 

 SPOEES), in a spore-case 

 which usually breaks in 

 pieces at maturity (Pig. 

 513-515). 



494. The Quill-worts, 

 Isoetes (Fig. 516-519), 

 are very unlike Club Mos- 

 ses in aspect, but have been 

 associated with them. They 

 look more like Hushes, and 

 live in water, or partly out 

 of it. A very short stem, 

 like a corm, bears a cluster 

 of roots underneath ; above 

 it is covered by the broad 

 bases of a cluster of awl- 

 shaped or thread-shaped 

 leaves. The spore-cases 

 are immersed in the bases 

 of the leaves. The outer 



leaf-bases contain numerous macrospores ; the inner are filled with innu- 

 merable microspores. 



495. The Pillworts (Marsilia and Pilularia) are low aquatics, which 



Fro. 520. Plant of Marsilia quadrifoliata, reduced in size ; at the right a pair of 

 sporo-carps of about natural size. 



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