Morphology of gymnosperms / by John M. Coulter and Charles J. Chamberlain.

  • Coulter, John Merle, 1851-1928.
Date:
[1910]
Catalogue details

Licence: In copyright

Credit: Morphology of gymnosperms / by John M. Coulter and Charles J. Chamberlain. Source: Wellcome Collection.

    33/494 (page 19)
    rather than intermediate between the Heterangium and Lyginoden- dron types. The polystely of Medullosa, however, is said (87) to differ from that of the ferns (such as Pteris), in that apparently it holds no relation to the presence of leaf gaps. The absence of leaf gaps in Medullosa and its allies has suggested that polystely arose within the family, quite independent of polystely among ferns. Occa- sionally vascular strands appear also in the cortex, as among the cycads. The leaf trace of Medullosa is at first concentric and with secondary wood; but later the secondary tissue disappears, the bundle branches repeatedly, and the branches become collateral and exarch. According to Scott (24), “each leaf stalk re- ceived a large number of vascular bundles, as many as 70 to 80 in all, derived from the repeated ramifi- cation of several of the principal leaf traces. The supply of bundles did not all enter the leaf base from the stem at the same level; they passed in successively in groups corresponding to the various principal leaf traces from which they were derived.” The later species of Medullosa, so far as their structure is known, are more complex anatomically than M. anglica, the vascular bundles being more numerous, and differentiated into an inner group of smaller bundles and a more or less complete peripheral zone of larger ones (fig. 16). All the bundles have the same general structure as those of M. anglica. In the older stems of certain forms, however, there is an excessive development of secondary wood on the outer side of the peripheral bundles, so that a section of the stem shows a broad zone of secondary wood surrounding the relatively inconspicuous inner region. In a specimen of M. stellata gigantea, whose vascular region is over 45 cm. in diameter, in addition to the large mass of secondary wood developed on the outer side of the peripheral bundles, there Fig. 16.—Medullosa Solmsii: part of transverse section of stele; st, “star rings”; pla, outer zone of bundles; pli, inner zone of bundles; a and b, bundles which have branched off from the inner zone; X2.—After Weber and Sterzel (17) in Potonie (20).