240 PLANT-LIFE 



in respect of the Calamites. Whereas to-day the tallest 

 species of Equisetum is not more than 20 feet high, and 

 at that must needs have support to maintain the per- 

 pendicular, so slender is its stem, there were among the 

 Palaeozoic Calamites species which attained the majesty 

 of forest trees. It is known that some of them grew 

 to a height of at least 100 feet, and had shafts up to 

 4 feet in diameter. In general habit they resembled 

 modern Horsetails. Their stems were hollow, and for 

 the most part were composed of tissue much softer than 

 is usual in trees ; but this softness was compensated for, 

 and the trees were rendered self-supporting by a develop- 

 ment of secondary wood, which does not occur in living 

 Horsetails, or, for that matter, in any plants lower than 

 the Gymnosperms. Whorls of branches grew from the 

 nodes of the stem, and alternating with these were 

 whorls of lanceolate leaves with their bases united into 

 a sheath. In Archceocalamites, a very ancient type 

 dating from the Devonian Period (see Fig. 76), the 

 leaves were more fully developed, having dichotomous 

 (two-forked) subdivisions. In some Calamites the cones 

 were in structure very similar to those of modern forms, 

 but in the majority they were more complicated, and 

 had, alternating with the sporophylls, whorls of scale 

 leaves. As a further mark of advanced development in 

 Palaeozoic times, it is noteworthy that some of the 

 Calamites produced mega- and micro-spores — i.e., they 

 were heterosporous. In this respect they differed from 

 modern Horsetails, which, it will be remembered, pro- 

 duce spores only of one kind. If the reader can imagine 

 in the country around him the existence of a goodly 

 number of Horsetail plants magnified to fifty times the 



