236 THE MICROSCOPE AND ITS REVELATIONS. 



surface of the water. This state seems to correspond with the ' winter- 

 spores ' of other Protophytes; and it may continue until warmth, air, and 

 moisture cause the development of the red cells into the ordinary ' still ' 

 cells, green matter being gradually produced, until the red substance 

 forms only the central part of the endochrome. After this, the cycle of 

 changes occurs which has been already described; and the Plant may pass 

 through a long series of these, before it returns to the state of the red 

 thick- walled cell, in which it may again remain dormant for an unlimited 

 period. Even this cycle, however, cannot be regarded as completing the 

 history of the Protococcus; since it does not include the performance of 

 any true Generative act. There can be little doubt that, in some stage 

 of its existence, a f conjugation ' of two cells occurs, as in Palmoglcea; 

 and the attention of observers should be directed to its discovery, as well 

 as to the detection of other varieties in the condition of this interesting 

 little Plant, which will be probably found to present themselves before and 

 after the performance of that act. 1 



235. Nearly related to the foregoing in the independence of their 

 individual cells, are the two groups Desmidiacece and Diatomacew, which, 

 in a systematic view, rank as subordinate divisions of the family Conju- 

 gates; their Generative process being performed in the same simple man- 

 ner as that of Palmoglcea, ( 229). But these two tribes being of such 

 special interest to the Microscopist as to require separate treatment ( 260), 

 only that higher group, the Zygnemacece, will be here noticed, in which the 

 cells produced by binary subdivision remain attached to each other, end 

 to end, so as to form long unbranched filaments (Fig. 141), whose length 

 is continually being increased by a repetition of the same process, which 

 may take place in any part of the filaments, and not at their ends alone. 

 The plants of this group are not found so much in running streams, as 

 in waters that are perfectly still, such as those of ponds, reservoirs, ditches, 

 or marshy grounds; and they are for the most part unattached, floating 

 freely or at near the surface, especially when buoyed-up by the bubbles 

 of gas which are liberated from the midst of them under the influence 

 of solar light and heat. In the early stage of their growth, whilst as yet 

 the cells are undergoing multiplication by subdivision, the endochrome 

 is commonly diffused pretty uniformly through their cavities (Fig. 141, A) ; 

 but as they advance towards the stage of conjugation, it ordinarily ar- 

 ranges itself into regular spirals (B), though occasionally in some other 

 forms. The act of conjugation usually occurs between the cells of two 

 distinct filaments that happen to lie in proximity to each other; and all 

 the cells of each filament generally take part in it at once. The adjacent 

 cells put forth little protuberances, which come into contact with each 

 other, and then coalesce by the breaking-down of the intervening parti- 

 tions, so as to establish a free passage between the cavities of the conju- 

 gating cells. In some genera of this family (such as Mesocarpus), the 

 conjugating cells pour their endochromes into a dilatation of the passage 

 that has been established between them; and it is there that they com- 

 mingle so as to form the ' zygospore.' But in the Zygnema (Fig. 141, B), 

 which is among the commonest and best-known of Conjugates, the 



1 In the above sketch, the Author has presented the facts described by Dr. Cohn, 

 under the relation which they seemed to him naturally to bear, but which differs 

 from that in which they will be found in the original Memoir; and he is glad to 

 be able to state, from personal communication with its able Author, that Dr. 

 Cohn's later observations have led him to adopt a view of the relationship of the 

 * still' and 'motile' forms, which is in essential accordance with his own. 



