172 



SEXUAL REPRODUCTION OF ULOTHRIX 



able. When new conditons arise, such as changes of tempera- 

 ture, food, etc., that would interfere with the normal growth of 

 the individuals then a modification of the contents of the cells 

 and its mode of division results. The cells of Ulothrix as a con- 

 sequence of such alterations divide into larger numbers of motile 

 bodies which are consequently smaller than in the case of zoo- 

 spores (Fig. 112, A). These bodies may behave as gametes and 

 unite, forming gametospores. The cells in which gametes are 

 formed are called gametangia, sing, gametangium. Thus, Ulo- 

 tlirix is controlled in the same manner as Chlamydomonas and 

 Hydrodyction. It is interesting to note, however, that these 

 small bodies frequently behave as zoospores and develop into 

 small and weak plants, which fact accounts for the common asso- 

 ciation in this genus of puny and vigorous plants (Fig. ill, 

 C,E). 



These small bodies of Ulothrix represent a curious intermediate 

 condition between a zoospore and a gamete where the sexual 



Fig. 112. Sexual reproduction of Ulothrix: A, a few cells of a filament 

 showing the formation and escape of the gametes. B, gamete. C, D, 

 stages in the union of the gametes. E, gametospore. F, gametospore 

 germinating and forming a single-celled plant in which several zoospores 

 arc developing. 



character is not strongly enough developed to overcome com- 

 pletely the zoospore character. Thus, we sec again in Ulothrix 



