66 THE HORSETAILS. 



tiun ari'eusc, it has been conjectured that these species 

 are its parents. In both structure and habit it is about 

 halfway between the two, and in some localities a series 

 of intergrading forms could be selected, leading almost 

 imperceptibly from one to the other. The strongest 

 point made against the theory of the hybridizing of 

 these two species is the fact that they do not fruit at 

 the same time, but in reality this maybe indirect evi- 

 dence in its favour. Equisetuin prothallia may live for 

 some months if unfertilized, giving ample opportunity 

 for crossing between species that do not fruit at the 

 same time, while the infrequency of such an occurence 

 would account for the rarity of the hybrid form. Since 

 in all the Eqniscta the sexes are on separate prothalli, 

 it would be a much easier matter to prove or disprove 

 this theory than it would be in the case of ferns where 

 both sexes are on the same prothallium. 



As commonly described, Equisctum littorale is three 

 feet or less in height, usually erect, with slender stems 

 containing from six to twenty grooves. In some speci- 

 mens the stems are unbranched, in others branched only 

 in the lower part, and in still others branches occur nearly 

 to the top. In all, the upper nodes are prolonged into a 

 whip-like tip which in fruiting specimens bears a small 

 fruit-cone. Usually the stems with cones are not 

 branched as much as the others. At the base of the 

 stem the sheaths are dark in colour; above, they are 

 green with short, narrow, dark-coloured teeth bordered 

 with white. The sheaths may be appressed near the 

 base of the stem, but beyond they usually widen out 

 The catkins are borne on very slender pedicels and often 

 do not become much larger than pepper-corns, though 

 occasionally they may attain a length of half an inch. 



