482 MOSSES AND FERNS chap. 



forms being so restricted in number and type, offer but partial 

 means of comparison ; still a comparison of these with the sim- 

 pler Filicinese does indicate some affinity between the two 

 groups, although, as might be expected, a very remote one. 

 Van Tieghem (6) has shown that the structure and arrange- 

 ment of the vascular bundles in the stem of Ophioglossiim and 

 Equisetiim have much in common. As we have seen, the pro- 

 thallium is not essentially different in Equisetum and the euspo- 

 rangiate Ferns, and the spermatozoids are closely like those of 

 the latter, and not at all like those of the Lycopodinese. This 

 latter point I believe to be one of great importance. 



If the Equisetinese do come from a common stock with the 

 Ferns, they must have branched off at a very remote period, 

 long before the latter had become completely differentiated. 

 The very different importance relatively of the stem and leaves 

 in the two groups points to this, as well as the extremely dis- 

 similar character of the sporophylls. The genus Equisetum 

 is evidently but a reduced remnant of a once predominant type 

 of plants which has been crowded out by the more specialised 

 Ferns and Spermatophytes. The presence of heterospory in 

 some fossil forms is interesting, but from what we know at 

 present it never developed to the same extent as in the other 

 groups of Pteridophytes. 



