THE FOSSIL FLORA OF THE YORKSHIRE COAL FIELD. 



41 



One is led, therefore, to believe that the primary vascular bundle in Sigillaria was 

 a closed vascular ring quite similar to that found in Lepidodendron ; but that in 

 Sigillaria, by the increase in the size of the pith core (which we know in Lepidodendron 

 increased in size up to a certain period), the original closed bundle became broken up 

 and carried outwards by the increasing volume of the pith, instead of persisting as a 

 closed ring, as in Lepidodendron. 



That the presence or absence of a secondary xylem is no proof of a Cycadaceous 

 affinity is abundantly shown in the case of Lepidodendron, which possesses a secondary 



B A C 



Sigillaria spinulosa. 



A, Primary vascular zone x 8 ; B, part marked a in tig. A, showing isolated bundles x 17 ; C, part marked 

 b in tig. A, showing union of primary bundles x 17 and formation of the Lepidodendron type of primary 

 xylem. This is further seen at d in fig, A. 



xylem, and whose fruit is in the form of cones, bearing macro- and microspores. But 

 this phenomenon of exogenously developed xylem is not entirely unknown amongst recent 

 Cryptogams, for "a secondary thickening, starting from a cambial layer, which pro- 

 duces secondary wood. and. secondary cortex, is found (only) in the Isoetes* 



* A. de Bary, Comparative Anatomy of the Vegetative Organs of the Phanerogams and Ferns, English edit., 

 p. 623, 1884. 



Note. — On the structure of Sigillaria and Lepidodendron the following additional works may be consulted : — H. 

 Graf zu Solms-Laubach, Fossil Botany, being an Introduction to Palceophylology from the Standpoint of the Botanist, 

 English translation, Oxford, 1891. Sir Wm. Dawson, "On the Conditions of the Deposition of Coal, more especially 

 Illustrated by the Coal Formation of Nova Scotia and New Brunswick," Quart. Journ. Geol. Soc, May 1866, vol. xxii. 

 p. 95. Ibid., Geological History of Plants, New York, 18S8. Ibid., Acadian Geology, 2nd edit., London, 1868. Sir W. 

 Hooker, " On the Vegetation of the Carboniferous Period, as Compared with that of the Present Day," Mem. Geol. 

 Survey of Great Britain, vol. ii. part ii. p. 387, 1848. Binney, Quart. Journ. Geol. Soc, 1862, vol. xviii. p. 106. Ibid., 

 Phil. Trans., vol. civ. pp. 579 and 591, 1865. Ibid., Pakeont. Soc, vol. xxix. pp. 97 and 147, 1875. P. van Tieghem, "Sur 

 tpaelques points de l'anatomie des cryptogames vasculaires," Bull. Soc. Bot. de France, vol. xxx. p. 169, 1883. Ibid., 

 TraiU de Botanique, 1884. Williamson, Memoirs, " On the Organisation of the Fossil Plants of the Coal Measures," 

 in Phil. Transactions, No. ii. 1872 ; No. iii., 1872 ; No. ix., 1878 ; No. x., 1880 ; No. xi., 1881 ; No. xii., 1881 ; No. 

 xvi., 1889. Ibid., " General, Morphological, and Histological Index to the Author's Collective Memoirs on the Fossil 

 Plants of the Coal Measures," part ii., Mem. and Proc. Mter. Liter, and Phil. Soc, vol. vii., series 4, session 1892-93, 

 1893. Ibid., " On the Light thrown upon the Question of Growth and Development of the Carboniferous Arbores- 

 cent Lepidodendra by a Study of the Details of their Organisation," Mem. and Proc. M'ter. Liter, and Phil. Soc, 

 session 1894-95, series 4, vol. ix., 1895. 



VOL. XXXIX. PART I. (NO. 5). I 



