74 EMBRYOGENESIS IN PLANTS 



bryophyte of which the sporophyte most closely resembles primitive 

 pteridophytes such as Rhynia; i.e. if we assume a common origin for 

 bryophytes and pteridophytes, Anthoceros may resemble the ancestral 

 type from which the pteridophytes have arisen. 



When the spore germinates, a filament is formed. This divides by 

 transverse walls. An apical cell is estabhshed and a thallus is formed as 

 a result of its regular segmentation. Fig. 17l. 



The archegonia in Anthoceros are formed behind the growing tip 

 of the thallus, Fig. 17a, and are completely embedded in it. In contrast 

 to other liverworts, the ovum is thus liable to be affected by biochemical 

 factors present in the surrounding tissue. At maturity the spherical, 

 or approximately spherical, ovum lies at the base of the venter, filling 

 only part of the space. On being fertilised, the ovum swells up; its 

 uniform, granular cytoplasm becomes highly vacuolated and fills the 

 cavity of the venter; and it secretes an outer wall. Fig. 17b. The nucleus 

 also increases in size. Thereafter, some interesting differences have 

 been observed in the development of different genera. In Anthoceros 

 fusiformis and A. pearsoni, Campbell shows the somewhat egg-shaped 

 zygote as dividing by a longitudinal wall into two equal parts, each of 

 these being divided transversely but unequally into a small basal cell 

 and a larger distal one. Fig. 17c, d. In each of the quadrant cells a 

 vertical wall is laid down and the octant stage is reached, the upper 

 cells being considerably larger than the lower ones. The upper cells 

 are next divided by transverse walls so that the embryo now consists of 

 three tiers each of four cells. On the further division and development 

 of the lower tier, which is considerably less regular, the characteristic 

 haustorial foot is formed, the constituent cells being typically enlarged 

 and parenchymatous, Fig. 17r. In these several developments in 

 Anthoceros it is evident that polarity is established from the outset — 

 though the first partition wall is not at right-angles to the axis— and 

 that the embryonic development is marked by a differential distribution 



Fig. 16. Reaction of gametophyte to embryonic development in 



Marchantiales and Jungermanniales 



A, B, Corsinia marchantioides. Longitudinal section of thallus. A, Shows out- 

 growth of thallus in midst of group of archegonia. B, Considerable development 

 of this outgrowth, associated with the development of the embryo (from Cavers, 

 after Leitgeb). C-E, Diagrammatic longitudinal sections of various Acrogynae, 

 showing the archegonial group and the sporogonium with the structures developed 

 round it. C, Perianth and involucre free, as in Lophocolea, Plagiochila, Frullaiiia. 

 D, Perianth and involucre fused; the formation of an incipient marsupium is indi- 

 cated by the bulbous swelling of the stem below the foot of the sporogonium, as in 

 NanJia geoscypha. E, Development of 'coelocauly,' i.e. the complete embedding of 

 the sporogonium in the stem tissue, as in Gottschea and Trichocolea. (C, D, after 

 Cavers; E, after Goebel). F, G, Kantia trichomaiiis. Stages in the development of 

 the hollow marsupium. (F, after Goebel; G, after Cavers); per perianth; inv, 

 involucre; c, capsule; i, seta; /, foot; gametophyte tissue stippled. 



