60 EMBRYOGENESIS IN PLANTS 



Kylin, 1917; Fritsch, 1945). The plantlings obtained from monospores 

 of Bangia fiiscopurpiirea. Fig. 12J-M, may be either of the germ-tube 

 type (the Naccaria-typQ of Chemin) or of the erect type, the reason for 

 the different kinds of growth not being understood. Thus, when Drew 

 (1952) cultured spores of this species, she found that at first the spores 

 gave rise to a creeping horizontally-growing filament, whereas later 

 germinations from the same material showed a gradual transition to the 

 upright type. 



The ontogeny of Nemalion has the added interest that, from the 

 simple beginnings illustrated in Fig. 12a-d, a multiaxial, cylindrical, 

 dichotomising and sometimes branching thallus is built up. Fritsch 

 regards the development of Nemalion as being essentially heterotrichous, 

 a basal system of short rounded cells giving rise to well-branched erect 

 filaments. In these developments, as also in the inception of differentia- 

 tion as between the central and cortical filaments of the cylindrical 

 thallus, there is scope for the investigation of the related morphogenetic 

 factors. 



The attachment-disc and hemispherical types of development are 

 well seen in such genera as ChylocJadia, CystocJonium, Fig. 12n, o, 

 Dumontia, Bonnemaisonia, Fig. 12u-w, and Chondrus. The spore, 

 surrounded by its gelatinous coat, becomes attached to the substratum 

 and, without increase in size, divides by a vertical wall and then by a 

 second vertical wall at right-angles to the first, Fig. 12n, u. Divisions 

 by horizontal walls and further vertical divisions follow. A centrally 

 placed cell on the upper side of the hemispherical embryo now becomes 

 a locus of growth and grows out to form the thallus, while cells along 

 the base develop as rhizoidal attachment structures, Fig. 12o, v, v/. 

 Contemplation of the high degree of regularity in the segmentation 

 pattern, and in the sequence of the developmental phases usual in these 

 organisms, leaves little doubt that we are here concerned with a true 

 embryogeny, comparable with that found in the brown algae and in 

 higher plants, and also, in some respects, with features found in the 

 development of some marine animals. Here it is appropriate to note 

 that the type of embryogeny described above may precede and give 

 rise to both uniaxial and multiaxial types of construction : CystocJoniiim, 

 Fig. 12n, o, RhodophyUis and Plocamium, for example, are specialised 

 uniaxial forms, while Chondrus, Gigartina, Chylocladia, Rhodymenia 

 and Lomentaria exemplify multiaxial construction. 



In the erect type of embryogeny, the spore elongates and divides by 

 successive transverse walls, the distal cell becoming the apical cell, 

 while the proximal cell develops as a rhizoid. Typical examples are 

 seen in the Ceramiales, e.g. Antithamnion, Fig. 6a-d, Ceramium, Fig. 

 lE, f; 12p-t, Polysiphonia, Figs. 2c; 12e-h, and Delesseria, Fig. 7a. 



