212 MOSSES AND FERNS chap. 



where they are remarkably uniform. In all of them the arche- 

 gonium has usually a neck composed of but four rows of per- 

 ipheral cells, instead of five or six, as in the Bryophytes, and the 

 antheridium, except in the leptosporangiate Ferns, is more or 

 less completely sunk in the tissue of the prothallium. The 

 spermatozoids are either biciliate, as in Mosses, or multiciliate, 

 a condition which, so far as is known, does not exist among the 

 Bryophytes. 



The formation of spores is very much more subordinated 

 to the vegetative life of the sporophyte than is the case among 

 the most highly organised of the Bryophytes. Indeed it may be 

 many years before any signs of spore formation can be seen. 

 The spores are always born in special organs, sporangia, which 

 are for the most part outgrow^ths of the leaves, but may in a 

 few cases develop from the stem. In the simplest cases the 

 spores arise from a group of hypodermal cells, generally trace- 

 able to a single primary cell. The cell outside of these divides 

 to form a several-layered wall, but the limits of the sporangium 

 are not definite, and it may scarcely project at all above the 

 general surface of the leaf. From this "eusporangiate" condi- 

 tion found in Ophioglossum, there is a complete series of forms 

 leading to the so-called leptosporangiate type, where the whole 

 sporangium is directly traceable to a single epidermal cell, and 

 where a very regular series of divisions takes place before the 

 archesporium is finally formed. 



With very few exceptions all of the existing Pteridophytes 

 fall naturally into three series or classes of very unequal size. 

 The first of these, the Ferns or Filicinese, is the predominant 

 one at present, and includes at least nine-tenths of all living 

 Pteridophytes. The Equisetinese are the most poorly repre- 

 sented of the modern groups, and include but a single genus 

 with about twenty-five species. The third class, the Lyco- 

 podinese, is much richer both in genera and species than the 

 Equisetinese, but much inferior in both to the Filicinese. The 

 disproportion between these groups was much less marked in 

 the earlier periods in the world's history, as is attested by the 

 very numerous and perfect remains of Pteridophytes occurring 

 especially in the coal-measures. At that time both the 

 Equisetinese and Lycopodinese were much better developed 

 both in regard to size and numbers than they are at 

 present. 



