. The origin of a land flora, a theory based upon the facts of alternation. Plant morphology. 56 LYCOPODIALES L. selaginoides. It would seem probable that the non-medullated condition, so persistently maintained in the smaller living Lycopods, was the primitive state also for the larger dendroid fossils. The other factor of expansion, by cambial activity, appears to have originated independently of medullation, since it occurs both in medullated and in non-medullated axes. Physio- logically it counterbalanced medullation where both occur together, for it. Fig. 176. Transverse section of an axi
. The origin of a land flora, a theory based upon the facts of alternation. Plant morphology. 56 LYCOPODIALES L. selaginoides. It would seem probable that the non-medullated condition, so persistently maintained in the smaller living Lycopods, was the primitive state also for the larger dendroid fossils. The other factor of expansion, by cambial activity, appears to have originated independently of medullation, since it occurs both in medullated and in non-medullated axes. Physio- logically it counterbalanced medullation where both occur together, for it. Fig. 176. Transverse section of an axis of Lepidodcndron selaginoides. Cy—centre of the vascular system ; /r=tracbeae; F=vessels of the primary cylinder ;_/^ = primilive fibres of the primary wood ; B»= tracheides of the secondary wood ; r=ray of the secondary wood ; ^*2==seconclary parenchyma ; 2e = cambial zone ; L = liber ; j= foliar traces detached from the primary cylinder. (After Hovelacque.) substituted an enlarging peripheral vascular supply for the reduction in efficiency in the limited central system. This was indeed a necessary condition for dendroid development. However large the proportion of pith to the primary wood became in Lepidodeiidron, the continuity of the ring was as a rule unbroken, and the leaf-traces were simply inserted upon the primary xylem with the minimum of local disturbance. But in Sigillaria, in which the leaves sometimes attained a very large size, the case is different : though they show in all. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Bower, F. O. (Frederick Orpen), 1855-1948. London, Macmillan and Co. , Ltd.
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