Asexual reproduction 15 



they are also specially cut off in the Vaucheriacese), and in the 

 filamentous Myxophycese hormogones or multicellular gemmae are 

 formed. Sometimes single non-motile cells are produced, which 

 have been termed by Wille akinetes when they are formed without 

 rejuvenescence and aplanospores when formed by rejuvenescence 

 (e.g. Chsetophorales, GWfervales, Conjugatse). Many of these non- 

 motile asexual spores, which may be either akinetes or aplanospores, 

 rest for considerable periods before germination and are known as 

 hypnospores (or sometimes as hypnocysts). Asexual reproductive 

 organs are known as gonidangia when borne on the gametophyte 

 generation and sporangia when borne on the sporophyte. A 

 sporangium (or a gonidangiurn) as a rule gives origin to a number 

 of spores (or gonidia), but in Vaucheria, (Edogonium, and some of 

 the Ulotrichacese only one gonidium is formed in the gonid- 

 angium. 



Very often the gonidia consist of motile cells which receive the 

 name of zoogonidia (or zoospores). One of the most frequent 

 methods of asexual reproduction is by means of zoogonidia, which 

 are found in all groups of freshwater Algas except the Conjugates 

 (the largest order of the Chlorophyceas), the Myxophycese, and the 

 Bacillarieae (or Diatoms). Zoogonidia are small masses of proto- 

 plasm formed singly by the rejuvenescence of the entire contents 

 of a cell, or more frequently in numbers by free cell-formation. 

 They are not possessed of a cellulose wall, but are furnished with 

 one (?), two, four, or many cilia, with one or more chromatophores, 

 and often with one or two contractile vacuoles. The cilia are 

 usually disposed towards one end or one side of the zoogonidium 

 and their rapid vibratile movements cause it to swim quickly 

 through the water. A red or brown pigment-spot is very often 

 present. After a time the zoogonidium comes to rest, the cilia 

 disappear, the protoplasm secretes a cellulose wall, and the zoo- 

 gonidium develops into a new plant. 



Sexual reproduction. Reproduction by the union of male 

 and female elements, or gametes, is fairly general throughout the 

 Algae, but it is entirely absent in the Myxophyceae, in some of the 

 unicellular Protococcoideaa, and in the Syngeneticse. Sometimes 

 the gametes are clearly differentiated into male and female ele- 

 ments, but in other cases sexual differences are scarcely appreciable. 

 The following is a summary of the sexual methods of reproduction 

 met with in the freshwater Alga^ : 



