18 Algce 



Many Algae exhibit an alternation of generations in their life- 

 history. In those forms in which neither asexual nor sexual 

 reproduction is known this alternation of generations is, of course, 

 absent. Other Algae exhibit slight indications of an alternation 

 of generations. Thus, in a large number of the Chlorophyceae, the 

 sporophyte generation is represented by the zygospore. This 

 zygospore can be described as a unicellular sporangium which often 

 gives rise to two or four zoospores. Each zoospore, on coming to 

 rest, germinates and produces the gametophyte generation. In 

 the Mesocarpeae the isogamous gametes conjugate in a connecting- 

 tube between the gametangia, and the zygospore immediately 

 produces a rudimentary sporocarp consisting of a variable number 

 of cells, one cell of which is the carpospore. This is the sporophyte 

 generation. The carpospore, after a period of rest, germinates and 

 gives rise to the new gametophyte generation. In the Rhodo- 

 phyceae there is a well-marked alternation of generations. 



In the Algae the gametophyte is the important generation ; 

 in fact, the 'plant ' is the gametophyte ; but as one proceeds higher 

 in the scale of vegetable life there is a great reduction of the 

 gametophyte accompanied by a corresponding increase in the 

 development of the sporophyte, until in the Phanerogams the sporo- 

 phyte becomes the 'plant' and the gametophyte is parasitic on it. 



POLYMORPHISM. 



Polymorphism occurs in most families of Algae. All those Algae 

 which exhibit an alternation of generations are polymorphic, and 

 some species appear to possess several different vegetative forms. 

 It is very doubtful, however, if polymorphism occurs in Algae to 

 the extraordinary extent advocated by some authors. Sirodot has 

 proved the occurrence of several types of structure in the genera 

 Batrachospermum and Lemanea, and it is fairly evident that the 

 plants at one time described under the genera Prasiola, Schizo- 

 gonium and Hormidium are different vegetative forms of one 

 genus. Polymorphism is well illustrated in Botrydium, and 

 numerous striking instances could be mentioned of other Algae in 

 which it occurs, notably in the common genus Pleurococcus. 



Hansgirg, and following in his footsteps Wolle, have endeavoured 

 to prove (on insufficient evidence) the existence of extraordinary 

 polymorphism in Algae, relegating most of the unicellular Algae as 



