HORSETAILS 207 



The fertile scales resemble those of a Horsetail 

 in form; they are shield-like, with the sporangia 

 underneath the shield. But hi the Calamite the 

 sporangia on each scale are only four in number. 



In some French forms, there are further elab- 

 orations, the fertile scales being enclosed in cham- 

 bers formed by outgrowths from the bracts above 

 and below them. 



In most of the cones of this type the spores, 

 which are often united hi fours (fig. 23, A), are of 

 one kind only, so far as observed, but some of the 

 spores are abortive (fig. 23, B). In one species, 

 however, there are spores of two kinds, in differ- 

 ent sporangia on the same cone. The megaspores 

 are about three times the diameter of the micro- 

 spores, so we have a not very advanced hetero- 

 sporous condition. It is interesting to find that the 

 large spores are accompanied by abortive spores 

 (see fig. 23, c). This has suggested the idea that 

 in this group of plants we can see how hetero- 

 spory began. The abortion of some of the spores 

 has eventually allowed the survivors to grow into 

 megaspores. The same result has been produced 

 artificially in recent experiments on some of the 

 Water Ferns. 



Anyhow, some of the Calamites were in ad- 

 vance of their successors in this point also, that 

 they had two kinds of spores, though the differ- 

 ence was not so strongly marked as in the hetero- 

 sporous Lycopods. 



