PAST HISTORIES OF PLANT FAMILIES 145 



any given specimen of Stigmaria had belonged to a 

 Lepidodendron or a Sigillaria. Between the two genera 

 there certainly existed the closest affinity and similarity 

 in general appearance. 



These two genera represent the climax of develop- 

 ment of the Lycopod family. In the Lower Mesozoic 

 some large forms are still found, but all through the 

 Mesozoic periods the group dwindled, and in the Ter- 

 tiary litde is known of it, and it seems to have taken 

 the retiring position it occupies to-day. 



CHAPTER XV 



PAST HISTORIES OF PLANT FAMILIES 



VIII. The Horsetails 



The horsetails of to-day all belong to the one genus, 

 Equiseht-m, among the different species of which there 

 is a remarkably close similarity. Most of the species 

 love swampy land, and even grow standing up through 

 water; but some live on the dry clay of ploughed fields. 

 Wherever they grow they usually congregate in large 

 numbers, and form little groves together. They are 

 easily recognized by their delicate stems, branching in 

 bottle -brush fashion, and the small leaves arranged 

 round them in whorls, with their narrow teeth joined 

 to a ring at the base. At the end of some of the 

 branches come the cones, with compactly arranged and 

 simple sporophylls all of one kind. In England most 

 plants of this family are but a few inches or a foot in 

 height, though one species sometimes reaches 6 ft., while 

 in South America there are groves of delicate-stemmed 



o 



plants 20 ft. high. 



The ribbed stems and the whorls of small, finely 

 toothed leaves are the most important external charac- 



(C122) 12 



