UNICELLULAR PLANTS. 263 



cumstance that when, passing from the cell form into the 

 still condition, the cilia disappear, the Y-shaped, or forked 

 internal portions remain visible. And it is then, also, that 

 the openings through the enveloping cell-wall become, for 

 the first time, visible. 



Perhaps the most remarkable of all the numerous 

 aspects presented by Protococcus pluvialis, is the form of 

 naked zoospores named byFlotow Hcematococcus porphyro- 

 cephalus. These are extraordinarily minute globules, con- 

 sisting of a green, red, and colourless substance in unequal 

 proportions. The colourless protoplasm in them, as in all 

 primordial cells, constitutes the outermost delicate boun- 

 dary; the red substance is for the most part collected 

 towards the anterior end in minute spherules ; the granular 

 green substance occupies more the under part, while the 

 middle is usually colourless. 



Propagation depends upon a division of the cell 

 contents, particularly of the colourless or coloured proto- 

 plasm, or of the primordial sac. This body, without any 

 demonstrable influence of a nucleus, is capable of sub- 

 dividing into a determinate number of portions. Each of 

 these acquires a globular figure, and in the next place 

 surrounds itself with an envelope of protoplasm, and then 

 represents a visible organism, which after the reabsorption 

 of the parent cell-membrane, is capable of existence as an 

 independent reproductive individual. Besides these, which 

 are the most usual modes of propagation viz. that of the 

 still- cells into two, and of the motile into four, secondary 

 cells there are a number of others which may be con- 

 sidered as irregular, and in which forms are produced 

 which do not re-enter the usual cycle until they have 

 gone through a series of generations. Sometimes, under 

 certain circumstances, the cell-contents of the still form 

 separate into eight or more portions, which become naked 

 zoospores t of small size (fig. 144 B.) It is not quite 

 clear what becomes of this form of motile zoospores, but 

 there seems reason for believing that they occasionally 

 develop an enveloping cyst, and thus become encysted 

 zoospores, and at other times secrete a cellulin tissue, 

 and become still-colls ; but most of them probably perish 

 without any further change. They would thus correspond 



