]52 ANCIENT PLANTS 



are very irregular. They are aborting members of the 

 tetrad, and appear to have been used as food by the 

 other spores. In each sporangium large numbers of 

 these tetrads develop and all the ripe spores seem to 

 have been of one size. 



In a species of Caianiites (C caskeana), otherwise 

 very similar to the common one we have been consid- 

 ering, there is a distinct difference in the sizes of the 

 spores from different sporangia. The small ones, how- 

 ever, were only about one-third of the diameter of the 

 large ones, so that the difference was very much less 

 marked than it was between the small 

 and large spores of the Lycopods. 



Among the palaeozoic members of 

 the group are other genera closely 

 allied to, but differing from Calamites 

 in some particulars. One of these is 

 Archceocalamites, which has a cone 

 „. T' . J almost identical with that of the living 



Fig. III. — Tetrads _, . • i ■ •! i 



of Spores of c<j/aOTz7« Equisetums, as It has no sterile bracts 

 s, Normal-sized spores; mingled with the umbrella-Hke sporo- 

 *;,^,&c., aborting spores, phylls. Other gcncra are more com- 

 plex than those described for Calamites, 

 and even in the simple coned Archceocalamites itself the 

 leaves are finely branched and divided instead of being- 

 simple scales. 



But no genus is so completely known as is Calamites, 

 which will itself suffice as an illustration of the palaeozoic 

 Equisetaceae. Though the genus, as was pointed out 

 above, shows several important characters differing from 

 those of Equisetum, and parallel to some extent to those 

 of the palaeozoic Lycopods, yet these features are more 

 of a physiological nature than a systematic one, and they 

 throw no light on the origin of the family or on its con- 

 nection with the other Pteridophytes. It is in the extinct 

 family dealt with in the next chapter that we find what 

 some consider as a clue to the solution of these problems. 



