XVl] SIGILLARIA 207 



penetration of the soil would expose them to an insufficient 

 supply of oxygen \ It is certain that Sigillaria had no tap-root, 

 but was supported on spreading subterranean organs bearing 

 spirally disposed long and slender rootlets which absorbed water 

 from a swampy soil. 



The regularity of the leaf-scar series on a Sigillarian stem 

 may be interrupted by the occurrence of oval scars with a 

 central scar and surrounding groove (fig. 193, E); these occur 

 in zones at more or less regular intervals on the stem, as seen 

 in the partially decorticated cast represented in fig. 199. 

 Zeiller has pointed out that the rows of oval or circular scars, 

 which mark the position of caducous stalked strobili, may occur 

 between the leaf-scars in vertical series, each of which may 

 include as many as 20 scars, while in other cases a single series 

 of such cone-scars may encircle the stem^. The zones are 

 usually of uneven breadth, as in S. Brardi, and their occurrence 

 produces some deformation of the adjacent leaf-scars. 



By the earlier writers Sigillaria was compared with suc- 

 culent Euphorbias, Cacti, and Palms; Brongniart' at first 

 included undoubted Sigillarian stems among Ferns, but after 

 investigating an agatised stem from Autun, he referred Sigil- 

 laria to the Gymnosperms* on the ground that it had the 

 power of producing secondary wood. It was then supposed 

 that Lepidodendron possessed only primary xylem, and that 

 the presence of a vascular meristem in Sigillaria necessitated 

 its separation from the lycopodiaceous genus Lepidodendron 

 and its inclusion in the higher plants. By slow degrees it was 

 recognised, as in the parallel case of the genus Calamites, that 

 the presence or absence of secondary vascular tissue is a 

 character of small importance. Williamson, whose anatomical 

 researches played the most important part in ridding the minds 

 of palaeobotanists of the superstition that secondary growth in 

 thickness is a monopoly of the Phanerogams, spoke in 1883 of 

 the conflict as to the affinities oi Lepidodendron and Sigillaria as 

 virtually over but leaving here and there "the ground-swell of a 



1 Cf. Prof. Yapp's account (08) of Fen vegetation. 



2 Zeiller (88) A. PI. lxxxv. ^ Brongniart (28) A. p. 63. 

 * Brongniart (39) A. ; (49) A. p. 55. 



