REPRODUCTION. 613 



the canal into the cavity of the other cell and then fuses with its 

 contracted protoplasm. In the Mesocarpeae and the Desmidieae 

 the protoplasm contracts simultaneously in both cells, and 

 both the masses travel into the canal ; so it is in the canal 

 that the fusion takes place and the spore is formed. 



The sexual process of the Diatomeae differs from that of the allied 

 Conjugatae, in that the protoplasmic contents of the two conjugating cells 

 escape from their respective cell- walls as a preliminary to fusion. It 

 represents a mode of the sexual process which is intermediate in character 

 between that of the typical Conjugatae and the conjugation of piano- 

 gametes. 



In comparing these modes of the sexual process with those 

 with which we have already become acquainted, we see at 

 once that the two cells in question constitute rudimentary 

 reproductive organs of the nature of gametangia, resembling 

 in this respect the cells of a Ulothrix-filament. But whereas 

 the protoplasm of the Ulothrix-cell undergoes division into a 

 number of portions which are set free as, at least potential, 

 planogametes, that of the cells of the Conjugatae does not 

 divide, but constitutes a single gamete. In speaking of the 

 sexual process in the Conjugatae, the process is termed conju- 

 gation, and the product a zygospore, on account of the general 

 similarity of the gametes. But it must not be overlooked 

 that this similarity is complete only in the Mesocarpeae and in 

 the Desmidieae. In the case of the Zygnemeae, it is clear that 

 the gamete which is formed first and which takes the more 

 active part in the process of conjugation must be regarded as 

 a male cell, whereas the other, formed later and passive in the 

 process of conjugation, must be regarded as a female cell. 



In the Florideae the sexual reproductive organs are differ- 

 entiated, but it is only the male organ which produces sexual 

 reproductive cells. In the simpler forms (Bangiaceae) the 

 male cells are developed several together from the protoplasm 

 of a single mother-cell, whereas in the higher forms each 

 mother-cell gives rise to only one male cell. The male-organ, 

 or antheridium, may consist of a single mother-cell (e.g. 

 Batrachospermum) or of a cluster or group of mother-cells. 

 The male-cells differ from the antherozoids and gametes 



