256 



PART IV. CLASSIFICATION'. 



form the gamete. The formation of the gamete takes place earlier in one 

 gametangium than in the other, and the first formed gamete travels 

 across the connecting channel into the cavity of the other gametangium 

 when it fuses with the other gamete ; the resulting cell surrounds itself 

 with a wall, and constitutes a zygospore. Since the first-formed gamete 

 is the more active in the process of conjugation, it may be regarded as a 

 male cell, the other as a female cell, so that there is a rudimentary differ- 

 entiation of sex. Further, since the cells of any one filament all behave 

 alike in the process of conjugation, it is possible to speak of male and 

 female filaments or individuals. In Zygogonium, however, the gametes 

 are similar, both as regards the time of their formation and their share 

 in conjugation ; in this form the gametes meet in the connecting channel 

 and there fuse to form the zygospore. 



After a period of rest, the zygospore germinates ; the outer coat is 

 ruptured, and the contents, covered by a thin cell-wall, protrude as a 

 filament which is divided by a transverse septum into two cells ; of these, 

 the one becomes elongated and remains narrow in the cavity of the spore, 

 undergoes no further division, and contains little or no chlorophj'll, 

 whereas the other becomes broader, contains one or more chloroplastids 

 and, by repeated division, forms a filament. Thus there is at first a 

 differentiation of the body into root and shoot, but this soon ceases to be 

 apparent. It is most clearly marked in Spirogyra. Principal genera : 

 Zygnema, Spirogyra. 



Spirogyra occurs as floating masses in ponds or slowly running water 

 during the warmer parts of the year, but only where the water is pure. 

 It may be found in conjugation about May or June. 

 Order 2. Ulothrichaceae. The unbranched filament is attached by a 



narrow elongated, frequently 



colourless, root-cell ; the growth 



in length of the filament is 



intercalary, that is, each cell 



elongates and divides by a 



transverse wall into two. 



The reproductive organs are 



quite undifferentiated ; any cell 



of the filament may become an 



asexual reproductive organ, or 



a sexual organ, a gametangium. 



In the former case the proto- 

 plasmic contents of the cell 



divide into two or four parts 



which are set free as zoospores ; 



in the latter case the contents 



divide into eight or sixteen 



which are set free as planoga- 



metes. The zoospores are some- 

 what pear-shaped in form, the 



more pointed end being colourless 



Fio. 146. Ulollirix zonota : A part of a fila- 

 ment from a cell of which planogametes are 

 escaping, the other cells having already emptied 

 themselves; B planogametes; C the process 

 of conjugation; D young zygospores; E mature 

 zygospore ; F germinating zygospore with 

 hyaline root ; G the contents of the embryonic 

 shoot dividing to form zoospores. 



