DIV. I 



PTERIDOPHYTA 



533 



like flowers were borne on the stem ; the sporangia were borne singly on the 

 sporophylls. Heterosporous. 



I 



FIG. 504. Lepidostrobus Vdtheimianus. 1, Transverse section of cone with microsporangia ; tetrads 

 to right below. 2, Cone in longitudinal section showing microsporangia above and macro- 

 sporangia below. 8, Transverse section of cone with macrosporangia. A, Macrospore in 

 longitudinal section. 5, Macrospore, probably opening in course of germination. (1-5 after 

 SCOTT, KIDSTON, BIXKEY.) From LOTSY. 



Order 6. Lepidodendraceae ( 132 - I33 ) 



The Lepidodendrons extend from the Devonian to the Rothliegende, but 

 are best developed in the Carboniferous period. They were tree-like plants 

 attaining a height of some 30 metres with dichotomously-branched stems which 

 grew in thickness. The leaves, which attained a 'length of 15 cm., were spirally 

 arranged and seated on rhombic leaf-cushions (Fig. 503). The cone-like flowers 

 (Lepidostrobus) were borne on the ends of branches or sprang from the stem 

 itself ; each sporophyll bore a single sporangium, which contained either macro- 

 spores or microspores (Fig. 504). The number of spores in the macrosporangium 

 was larger than in Selaginella. A prothallus was formed in the spore and 

 resembled that of the existing genus. 



The discovery of seed-like structures borne by some of the palaeozoic 

 Lycopodinae (Lepidocarpon, Miadesmia) is of special interest. In them the 

 macrosporangium was surrounded by an integument leaving only a narrow slit- 

 like opening ; the sporophyll also took part in enclosing the sporangium. Only 

 one macrospore attained full development. As in Isoetes the prothallium re- 

 mained within the spore. The macrospores were produced on sporophylls 

 resembling those of Lepidostrobus. Probably pollination occurred while the 



