Chapter IV 

 EMBRYOGENESIS IN THE ALGAE 



THE plants of the land have been described collectively as the 

 Embryophyta, the implication being that in these plants there is a 

 characteristic embryonic phase which is not present in the great aquatic 

 groups of the algae. If, however, we regard as an embryo any small 

 germ which develops from a fertilised egg or from a spore, then it is 

 clearly legitimate to include the algae in this survey. This, indeed, is 

 fully justified by the facts, for in algae such as Fucus there are develop- 

 ments which are very similar to those in archegoniate plants. It might 

 perhaps seem permissible to apply the term embryo to the young algal 

 plant which develops from a fertilised ovum, but not to the plantling 

 that develops from a spore. This procedure, however, would bring its 

 own difficulties in the isomorphic green, brown and red algae. 



The ontogenetic development of the algae affords many points of 

 interest. Thus a majority of the green algae never advance beyond the 

 filamentous stage, whereas others, such as the large brown and red 

 algae, pass through an embryonic phase and proceed to one of con- 

 siderable somatic elaboration, i.e. their ontogenetic development is 

 comparable with that of land plants. In such comparisons the Chaeto- 

 phorales are of special interest. It has been suggested (Fritsch, 1939, 

 1945) that it is from this group of green algae that the plants of the land, 

 i.e. the Embryophyta, have originated. 



GREEN ALGAE 



Haplobiontic green algae have no embryonic phase that is 

 strictly comparable with that found in organisms in which the sporo- 

 phyte has its inception in a fertilised egg. In algae like Spirogyra 

 or Oedogonium, the zygote (zygospore or oospore), after a period of 

 rest, again becomes active, its diploid nucleus undergoes meiosis, and 

 one to four haploid propagative cells are formed. Where these are 

 motile, the zoospore eventually settles down, attaches itself to a sub- 

 stratum, and grows out into a filament. In Spirogyra, Figs. 2, 8, a 

 filament grows directly out of the zygospore which has split open. In 

 this, as in other filamentous green algae where the plantling is initially 

 attached to the substratum, the germ shows evidence of polarity from 

 the outset. The distal region is the locus of active growth (though in 

 some species intercalary growth may subsequently supervene), and, as 



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