312 



REPRODUCTION OF EOUISETUM 



genus as a whole is derived from aquatic ancestors. Indeed 

 there is good e\idence that the Calamites of the Coal Measures, 

 which exhibited similar aquatic features combined with pro- 

 nounced secondary thickening, were inhabitants of swamps. 



The sporangia of Horsetails are borne on little mushroom- 

 shaped sporophylls (Fig. 176, B), differing widely from the 



Fig. 175. — Common forms of prescr\'ation of the fossil Horsetails and 

 Clubmcsses. A, Pith-cast of Caltniules, B, Annularia sphenophyl- 

 loides. the foliage of Calamiles. C, Small part of the surface of the 

 stemof a Ic/>)rforf(;»c'ro», showing the characteristic Icaf-cnshions [I.e.) ; 

 I.S., leat-scar. 



ordinary lca\'es, and collected together at the ends of the stems 

 to form cones or strohili (Fig. 174, ]i ; Fig. 176, A). These 

 usually terminate the ordinary vegetative shoots, though in the 

 Common Field Horsetail (E. arvcnsc) they are found on special 

 fertile shoots, produced earlier in the season, and distinguished 

 by their brown colour and the absence of branches. Each 

 peltate sporophyll stands out at right-angles to the main axis, 



