THALLOPHYTA— ALG^ 



37 



of the chromatophores is very constant in the several species. 

 In some of them curious bodies called pyrenoids are found, 

 which are probably connected with the assimilative processes. 



The group exhibits considerable variety in its modes of sexual 

 reproduction. In the simplest forms that show sexuality, the 

 gametes are not distinguishable into male and female. In 

 Ulothrix (fig. 803) the contents of some cells break up into a 

 number of ciliated masses of protoplasm which escape from 

 the cell, -and, after swimming about for a while, conjugate in 

 pairs. In the Zygnemese and Mesocarpese the gametes are 

 solitary and not motile, and do not escape from the cells in 



Fig. 787. 



Fig. 786. 



Fig. 788. 



Fig. 786. Antlieridia, a, a, on the branched hairs of the male oonoeptacle. 



After Thuret. Fig. 787. Oogonium with the oospheres fully separated, 



and disengaging themselTes from their coTerings. After Thuret. 



Fig. 788. An oosphere without a cellulose coat being fertilised by anthe- 

 rozoids so as to form an oospore. 



which they are formed. In Cutleria the gametes are dissimilar 

 in size, but both are ciliated. The larger comes to rest soonest, 

 and one of the smaller fuses with it. More completely differen- 

 tiated gametes are found in higher forms (figs. 787 and 789) ; 

 oospheres are developed in oogonia and antherozoids ia an- 

 theridia. When the gametes are alike the reproduction is called 

 isogamous ; when they are different in size and behaviour it is 

 said to be oogamous. In the Bhodophyoes the antherozoid is 

 not ciliated and there is no differentiated oosphere. Instead of an 

 oogonium the female organ is known as a carpogonium. It is 

 frequently multicellular, and fertilisation is brought about 

 through a filiform or elongated cell known as a trichogyne. 



