PTERIDOPHYTA. 



221 



Class 2. Equisetinae (Horsetails.) 



The characteristics of this class have been described on page 

 204. 



It is divided into two sub-classes : 



1. THE ISOSPOROUS EQUISETINJE. To this sub-class belong, with 

 certainty, only the EQUISETACE^E now existent, which are repre- 

 sented by only one genus, Eqtiisetum. 



2. The HETEROSPOROUS EQUISETINAE. Forms which are now ex- 

 tinct. 



Sab-Class 1. Isosporous Equisetinae. 



Order. Equisetaceae (Horsetails). 



The sexual generation. The pro thallium is green and leaf- 

 like, as in the majority of Ferns, but irregularly branched and 



Fio. 222. Equi*etum arvense. Tde prothallium highly 

 magnified. A Male ; s, s autheridia. B Portion of a 

 female, cut through vertically ; ce ce archegonia, the central 

 one is fertilised ; h h root-hairs. 



FIG. 223.- EquifetummaX' 

 tmttm. Spermatozoids : a 

 shows them still enveloped 

 by the mother-cell. 



curled. It is often unisexual. The male pro thallia bear antheridia 

 only, and are smaller and less branched (Fig. 222 A) than the 

 female ; the latter may attain a diameter of | an inch, and bear 

 archegonia only (Fig. 222 B). The antheridia and the archegonia 

 resemble those of the Ferns, but the spermatozoids (Fig. 223) are 



