68 EMBRYOGENESIS IN PLANTS 



photoperiodic effects, or other causes, or that particular genes, in a 

 consistent and orderly sequence, determine the highly regulated develop- 

 ment of these organisms. Alternatively, the regulated development may 

 be due to factors of some other kind, as yet unknown. To attempt to 

 describe the observed developments in terms either of metabolism or of 

 genie action, both of which are undoubtedly involved, is merely to 

 indicate how very little is known about these phenomena. 



In several genera, e.g. Grimaldia, Plagiochasma, and in Conoceph- 

 alum conicum Fig. 14, the zygote, after filling the venter and dividing 

 transversely, continues to elongate and divides by several transverse 

 walls. The short, thick filament thus formed, consisting of 3 or 4 cells, 

 is also a very characteristic feature of the embryogeny of the Sphaero- 

 carpales. Fig, 14k. After some further transverse divisions, longitudinal 

 divisions appear towards the middle of the embryo. Oblique divisions 

 of the distal cell result in the establishment of a two-sided apical cell, 

 Fig. 14. 



Meyer (1931 ; see also Miiller, 1951) considers that the filamentous 

 and quadrant types (as in Marchantia) are quite distinctive, the former 

 being the more primitive and the latter derivative. On this basis he 

 recognises two types of embryo in the Marchantiales, each including 

 both relatively simple and primitive forms and more highly differen- 

 tiated ones. Fig. 13. Among the filamentous types are Plagiochasma, 

 Conocephalum, Grimaldia, Reboulia, and Fimbriaria and probably also 

 Cryptomitrium and Cyathodium, while the quadrant type is exemplified 

 by Marchantia, Conocephalum, Preissia and by members of the 

 Astroporae, Ricciaceae and Corsiniaceae. Conocephalum conicum may 

 show either the quadrant or filamentous type of development. 



The analysis of the problem of segmentation pattern can be taken 

 a stage further. There is support for the view that, at any particular 

 stage in development, the segmentation pattern is determined by the 

 shape of the organism, or organ, and by the internal distribution of 



Fig. 14. Embryogeny in Marchantiales and Sphaerocarpales 



A-D, Conocephalum conicum {Fegatella conica) (after Cavers). A, Embryo at 

 octant stage, showing the first and second partition walls. B, Similar embryo as 

 seen in transverse section. C, D, Older embryos, showing the regular segmentation 

 pattern ( x 360). E-G, Targionia hypophylla. E, Venter of mature archegonium 

 with ovum. F, The first division of the zygote. G, Regular segmentation pattern 

 (E, X 500; F, G, x 500). H, J, Riccia glaiica. H, First division of the zygote, as 

 seen in longitudinal section. J, Older embryo (x 260). K, Sphaerocarpiis sp. 

 Young embryo in l.s.; the fertilised egg elongates and divides transversely, and 

 further transverse walls are formed before any longitudinal ones appear ( x 260) 

 (E-K after Campbell). L-Q, Conocephalum conicum (after Meyer). L, Unfertilised 

 egg (x 900). M, Two-celled embryo (x 133). N, Second division of the zygote 

 (x 200). O, Four-celled embryo (x 200). P, Anomalous division in four-celled 

 embryo (x 200). Q, Older embryo with apical cell (x 133). 



