ORGANIZATION OF THE FOSSIL PLANTS OF THE COAL-MEASURES. 739 
3, Secondary Tissues of the Root. 
Among the numerous specimens of roots of Lyginodendron which we have 
examined, we find the secondary tissues in every stage of development, from their 
first appearance (see Plate 20, photograph 12 and Plate 25, fig. 20) up to a stage when 
the secondary wood has attained a radial thickness of about 20 cells (see photo- 
graph 14). The process takes place exactly in the manner typical for roots of 
Dicotyledons, so that this fossil might very well be used for purposes of demonstra- 
tion as illustrating the secondary growth of a root with diagrammatic clearness, 
The cambium first appeared immediately within the groups of phloém, arising 
from the division of the conjunctive cells lying in that position. It thus formed at 
first a series of isolated arcs separated from one another by the protoxylem-groups. 
This is well seen in fig. 20 which shows the first layer of wood formed from the 
cambium, cb,* It is probable, that in the meantime a few additional traches, the 
‘“metaxylem” of Van TrecHEM (loc. cit., p. 684), had been differentiated from the 
conjunctive tissue between the primary groups of xylem (cf. photograph 11 with 
photograph 12). 
The cambial divisions soon spread to the pericycle outside the protoxylem-elements 
(see photograph 12) so as now to form a complete though not a circular ring. The 
whole ring, however, did not behave alike. (Opposite the primary phloém, secondary 
xylem consisting of tracheze with narrow secondary rays was formed, while outside 
the primary xylem, parenchyma, constituting the large principal rays, was at first 
chiefly developed.t 
We thus find the secondary tissues broken up into distinct bundles corresponding 
in number and position to the primary groups of phloém and separated from one 
another by the principal rays, which correspond to the protoxylem-strands. In fact, 
the mode of secondary growth is exactly that which is characteristic of a large 
number of roots among recent Dicotyledons.{ Among Gymnosperms, curiously 
enough, we find a less close analogy, though in some of the finer roots of Cycads, as, 
for example, in rootlets of Stangeria, the process is essentially similar. 
Sooner or later the principal rays often become sub-divided by the intercalation of 
additional strands of xylem and phloém (see especially photograph 14). 
The details of the secondary wood require no special description. It is composed of 
tracheides and rays exactly as in the stem. The pits of the tracheides are usually 
limited to the radial walls.§ 
* See also Wirtiamson, “Organization,” Part XIIL, Plate 24, fig. 27; ¢f. Van Tizcuen’s figure of the 
root of Piswm, “ Traité de Botanique,” second edition, p. 722, fig. 481. 
+ See photographs 13 and 14, also Witutamson, “ Organization,” Part VII, Plate 5, figs. 23 and 26; 
Part XIII., Plate 23, fig. 26. 
t See pz Bary, ‘Comparative Anatomy,” English edition, p. 474; Van Trmanem, ‘Ann. des Sci. 
Nat., Bot.,’ Series V., vol. 18, 1871, p. 274. 
§ Wituiamsoy, “Organization,” Part VII., Plate 4, fig. 29; Plate 5, fig. 27; Plate VI, fig. 30. 
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