FACTORS IN EMBRYOGENESIS 9 



is of brief duration. Such a phase is conceivable and may indeed have a 

 real existence, e.g. in the free-floating zygotes of certain algae such as 

 Fucus. Where the embryo is enclosed, as in the Embryophyta, although 

 occasional aberrant developments have been recorded, the normal 

 embryogeny yields evidence of being closely controlled, or regulated, 

 from the outset. 



The filamentous or axial development, as illustrated in Fig. 1, is 

 characteristic of the embryogeny in all classes of plants and has been 

 described by Bower (1922) as constituting a 'primitive spindle.' This 

 development is attended by cell division and the laying down of partition 

 walls, usually in a very regular and constant manner, so that the embryo 

 has a characteristic cellular pattern from the outset. The first partition 

 wall is typically laid down at right-angles to the axis of the embryo, 

 i.e. to the direction of elongation of the primitive spindle or filament. 

 In some embryos several successive transverse divisions take place and 

 an elongated filament results : in others, divisions at right-angles to the 

 first wall take place, and in this we see the inception of a tissue mass. 

 In many of the smaller algae the organism never develops beyond the 

 filamentous state; in others, a flat thallus may be formed; but in the 

 larger algae, especially the brown, and in archegoniate and seed plants, 

 the distal region of the filament sooner or later develops into a multi- 

 cellular tissue. At this stage another important general phenomenon 

 becomes evident, namely, that the most distal region is the locus of 

 active growth. This can be observed in many filamentous algae and 

 in all classes of parenchymatous plants : it is a feature of the embryo- 

 geny of Fucus and Polysiphonia ]usi as it is of Lycopodium and Capsella. 

 On the further development of these embryos it can be seen that the 



Fig. 1. Young embryos from different systematic groups 

 A, Enteromorpha intestinalis, a green alga: young plant from zoospore (after 

 Kylin). B, Fritschiella tuberosa, a green alga : filament showing beginning of paren- 

 , chyma formation. C, Laminaria digitata, a brown alga: young sporophytes still 

 attached to oogonia. D, Fucus vesiculosus, a brown alga (after Thuret and Olt- 

 manns). E, F, Ceramium areschoughii, a red alga: young plants from spores (after 

 Oltmanns). G, H, Radula complanata, a liverwort (Jungermanniales) : young sporo- 

 phytes (after Leitgeb). J, Aneura multifida, a liverwort (Jungermanniales) : young 

 sporophyte (after Leitgeb). K, Fegalella {Conocephalum) sp., a liverwort (Mar- 

 chantiales): young sporophyte (after Cavers). L, Funaria hygrometrica, a moss: 

 young embryo (after Campbell). M, Lycopodium annotinum, a lycopod (after 

 Bruchmann). N, O, Selaginella spinulosa, a ligulate lycopod: young and older 

 embryos (after Bruchmann). P, Adiantum concUmum, a leptosporangiate fern: 

 embryo in post-octant phase (after Atkinson). Q, Sequoia sempervirens, a gymno- 

 sperm : two embryos borne on primary suspensors, with embryonal cells beginning 

 to form secondary suspensors (after Buchholz). R, Lobelia amoena, a dicotyledon: 

 filamentous embryo (after Hewitt). S, T, Sagina procumbens, a dicotyledon: two 

 stages in the development of the embryo (after Soueges). U, V, Nicotiana sp., a 

 dicotyledon (after Sou^ges). W, X, Poa annua, a monocotyledon: two stages in 

 development (after Soueges). Y, Daucus carota, a dicotyledon (after Borthwick). 



