246 PART III. THE CLASSIFICATION OF PLANTS. 



continuous. The protoplasmic contents of each cell contract away from the 

 wall of each gametangium to 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 differentiation 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 indi- 

 viduals. 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. 



Occasionally bodies resembling zygospores are formed in the cells of a fila- 

 ment without any conjugation; these are distinguished as azygoypores, and are 

 products of parthenogenesis (see p. 87). 



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 chlorophyll, whereas the other becomes 

 broader, contains one or mote 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 and Sirogonium (see Fig. 6). Principal genera : Zygnema, Spirogyra 

 (incl. Sirogonium), Zygogonium. 



Family 3. Mesocarpea. These plants resemble the Zygnemeae in all the 

 chief features of structure and reproduction, but can be distinguished by the 

 following peculiarities. In the first place there is no contraction of the whole 

 piotoplasrnic contents of the gametangium to constitute a gamete, but a portion 

 only of the contents of the two (or sometimes three or four) cells fuse ; secondly, 

 conjugation always takes place so that the product of conjugation lies between, 

 and not in, either of the gametangia ; thirdly, the product of conjugation is not 

 at once a zygospore, but is a mass of protoplasm which becomes shut off from 

 the conjugating gametangia by the formation of a septum on each side; this 

 mass of protoplasm then surrounds itself with a proper wall and proves itself 

 to be a spore by germinating. There is no marked differentiation of the young 

 plant into root and bhoot. Azj'gospores are of frequent occurience in this 

 family (esp. Gonatonema). Genera : Mougeotia, Gonatonema. 



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 organ, a gonidangium, or a sexual organ, a gametan- 

 gium. In the former case the protoplasmic contents of the cell divide into two 

 or four which are set free as zoogonidia ; in the latter case the contents divide 

 into eight or sixteen which are set free as planogametes. The zoogonidia are 



