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 l.s., 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 



