. Embryogenesis in plants. Embryology. 86 EMBRYOGENESIS IN PLANTS as a cylindrical and somewhat irregularly bifurcating rhizome. Sub- sequently, on exposure to light, some of its branches become erect and photosynthetic (Holloway, 1917, 1921, 1939; Lawson, 1917).. Fig. 19. Tmesipteris tannensis A, Transverse section of prothallus, showing archegonium {a) and young embryo in longitudinal section; the zygote has divided by a median wall; the epibasal seg- ment will give rise to the shoot {s) and the hypobasal segment to the foot (/). B, An undivided egg and a zygote showing somewhat irregular di
. Embryogenesis in plants. Embryology. 86 EMBRYOGENESIS IN PLANTS as a cylindrical and somewhat irregularly bifurcating rhizome. Sub- sequently, on exposure to light, some of its branches become erect and photosynthetic (Holloway, 1917, 1921, 1939; Lawson, 1917).. Fig. 19. Tmesipteris tannensis A, Transverse section of prothallus, showing archegonium {a) and young embryo in longitudinal section; the zygote has divided by a median wall; the epibasal seg- ment will give rise to the shoot {s) and the hypobasal segment to the foot (/). B, An undivided egg and a zygote showing somewhat irregular divisions. C, An older embryo in , showing the segmentation pattern in the shoot and foot segments. D, An older, leafless, rootless embryo which is about to dichotomise; two apical cells (ac) can be distinguished. E, A zygote unequally divided by the first wall. F, A zygote showing anomalous segmentation. G, An embryo showing the character- istic bifurcation. H, The foot region (/) of an older embryo; note the haustorial outgrowths and the starch grains in the cells of the gemetophyte and foot. (A-E, H, X 50; F, X 90; G, x 27; after Holloway). In both Psilotum and Tmesipteris the young sporophyte is rootless and leafless and Bower (1935) has pointed out that the whole character of the early embryonic development 'suggests an imperfect differen- tiation of parts, which in consideration of the adult structure may be held to be a relatively primitive rather than a reduced state of organisa- tion.' As we have seen, true leaves are never formed in Psilotum but small scale-like emergences of the shoot may be observed. In Tmesipteris. 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 Wardlaw, C. W. (Claude Wilson), 1901-. London, Methuen; New York, Wiley
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