46o MOSSES AND FERNS chap, xiii 



spores is much less than in the living heterosporous Filicineae 

 or Lycopodineae. 



The Calamarieae are all very ancient types. The first 

 certain remains of them occur in the Upper Devonian, and they 

 disappear before the Trias.^ 



Affinities of the Equisetinecs 



The Equisetineae, as will be seen from the account of the 

 fossil forms, are a very ancient group, and their relation to the 

 other Pteridophytes somewhat problematical. The modern 

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

 means of comparison ; still a comparison of these with the 

 simpler Filicineae does indicate some affinity between the two 

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

 Van Tieghem ^ has shown that the structure and arrangement 

 of the vascular bundles in the stem of Ophioglossum and 

 Equisetum have much in common, and a careful study of the 

 development of the bundles in the young sporophyte of the 

 latter may perhaps show still further resemblances. As we have 

 seen, the prothallium is not essentially different in Equisetum 

 and the eusporangiate Ferns, and the spermatozoids are closely 

 like those of the latter, and not at all like those of the Lyco- 

 podineae. This latter point I believe to be one of great 

 importance. 



If the Equisetines 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 

 dissimilar 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 Spermaphytes. The presence of hetero- 

 spory is interesting, but from what we know at present it 

 never developed to the same extent as in the other groups of 

 Pteridophytes. 



^ Solms-Laubach (2), p. 322. ^ Van Tieghem (6). 



