GBLL FORMATION BY UNION. 



45 



iBstead of being independent, as in the previous case. At 

 the time of union the filaments approach one another and lie 

 nearly parallel ; protuberances grow out from the contiguous 

 cells (Kg. 36, a, b) ; their extremities meet, and the walls are 

 absorbed, making a channel of communication from cell to 

 cell (Fig. 36). Through this channel the protoplasm from 

 one of the cells passes into the cav- 

 ity of the other ; the two masses 

 unite and form a round or ovoid 

 cell, which soon secretes a wall of 

 cellulose (Fig. 37, A, b, and B, c). 



The particular kind of union in which 

 the two cells are of equal or nearly 

 •equal size, and illustrated ahove by Cos- 

 marium and Spirogyra, has received the 

 name of Conjufration. It is character- 

 istic of one group of the Tliallophytes, 

 viz., the Zygosporem. 



59. — In Vaucheria, a fresh-wa- 

 ter Thallophyte, we have an ex- 

 ample of the union of cells of very 

 different sizes. The larger cells 

 (called oospheres) are in lateral 

 protuberances of the large single 

 cell which composes the whole 

 plant (Fig. 38, A, and B, og). The 

 protoplasm in these is of a spheri- 

 cal form, and is much denser than 

 in the main cell, from which it is 

 ■separated in each case by a trans- 

 verse wall (shown in F). The 

 smaller cells (the spermatozoids) 

 are produced by the internal cell- 

 division of the protoplasm of simi- 

 lar protuberances (the antheridia, ^,.and B, a). They are 

 very small as compared with the oospheres, and are naked 

 masses of protoplasm provided with two cilia, by means of 

 which they are locomotive {D). Upon escaping into the 

 water by the bursting of the old wall, they swim about, and 



Fig. 36.— Two filaments of Spiro- 

 gyra longata about to conjugate ; 

 at a and S are seen the protuber- 

 ances from the contiguous cells 

 approaching each other. X 550. — 

 After Sachs. 



