382 



BOTANY OF THE LIVING PLANT 



It is a pear-shaped, nucleated protoplast, with two cilia attached 

 laterally so that one is directed forwards the other backwards. They 

 are inserted close to a red eye-spot, which as a rule is closely related 

 to a yellow chromatophore (Figs. 284, B ; 287, 3). The constancy 

 of this type shows the probable phyletic unity of the Brown Algae. 



Fig. 284. 



A, Pleurocladia lacustris. Uni- 

 locular sporangium with its contents 

 divided up into zoospores, a = eye-spot. 

 chr = chromatophore. (After Klebahn.) 

 B= Chorda filum, zoospores. (After 

 Reinke.) (From Oltmanns' Algae.) 



Fig. 285. 



Ectocarpus siliculosus. i, female gamete sur- 

 rounded by a number of male gametes. 2-5, stages 

 in the fusion of gametes. 6, zygote after 24 

 hours. 7-9, fusion of the nuclei as seen in fixed 

 and stained material. (1-5 after Berthold ; 6-9 

 after Oltmanns.) (From Strasburger.) 



In the simplest of the Phaeophyceae, the Ectocarpales, the complete life- 

 cycle, as in Ectocarpus or Pylaiella, involves an alternation between (i) a diploid 

 plant bearing asexual, unilocular sporangia, Fig. 284, in which reduction 

 division takes place during the formation of the zoospores, and (ii) a haploid 

 plant, bearing plurilocular sporangia ( =gametangia) from which haploid 

 isogametes are liberated and fuse in pairs, Fig. 285. Thus, a zoospore from a 

 unilocular sporangium gives rise, on settling and germinating, to a haploid 

 gametophytic plant ; a zygote on germination gives rise to a diploid sporo- 

 phytic plant. The haploid and diploid plants are so similar in size and struc- 

 ture as to be described as identical ; hence they may be considered to exemplify 

 homologous or isomorphic alternation. Both phases of the life-cycle have 

 accessory means of vegetative propagation : diploid plants may be propagated 

 over several generations by diploid zoospores released from plurilocular 

 sporangia ; haploid plants may be similarly propagated by swarmers (apo- 

 gamous gametes) from the plurilocular sporangia or gametangia. The sporo- 

 phytic plant is always recognisable as that which bears unilocular sporangia 



