CYANOPHYCE^: ; CONJUGATE 549 



the bluish-green scum which floats on the surface of stagnant water. 

 Colonies of species of Xostoc and Anabcena are frequently endophytic 

 within the cells of Marchantia and other Hepaticre, the prothallia of 

 ferns, or other aquatic or moisture-loving plants. 3/~ostoc multiplies, 

 like the Oscillatoriacese, by the subdivision of its filaments, portions 

 of which escape from the gelatinous mass w T herein they were 

 imbedded, and move slowly through the water in the direction of their 

 length. These are ' hormogones,' similar to those of the Oscilla- 

 toriacese. After a time they cease to move, and a new gelatinous 

 envelope is formed around each piece, which then begins to increase 

 in length by the transverse subdivision of its segments. By the 

 repetition of this process a mass of new filaments is produced, the 

 parts of which are at first coniiised, but afterwards become more 

 distinctly separated by the interposition of the gelatinous substance 

 developed between them. Besides the ordinary cells of the beaded 

 filaments, two other kinds are known, both larger than the ordinary 

 cells, and called respectively heterocysts and resting -spores. The 

 function of the former is unknown ; the latter develop directly into 

 new individuals by division in the transverse direction only, with- 

 out any sexual process. 



Resembling the Protococcacese in the independence of their 

 individual cells are the two groups Desmidiacece and Diatomacece, 

 forms of such special interest to the microscopist as to require 

 separate treatment, and a detailed description of which will be found 

 later on. The Desmidiacece constitute a group of the family 

 Conjugatae, so called from their mode of reproduction by conjugation, 

 a process best exemplified in the higher group, the Zygnemacece, in 

 which the cells produced by binary subdivision remain attached to 

 each other, end to end, so as to form long unbranched filaments 

 (fig. 420), 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, of reservoirs, ditches, bogs, or marshy grounds ; 

 and they are for the most part unattached, floating freely at or 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 division, the endochrome is 

 frequently diffused pretty uniformly through their cavities (fig. 420, 

 A) ; but as they advance towards the stage of conjugation, it 

 ordinarily arranges itself into regular spirals (B, Spirogyrd), a couple 

 of star-like discs in each cell (Zygnema), or a single plate running 

 through it in an axile direction (Mesocarpus) . 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 partitions, so as to 

 establish a free passage between the cavities of the conjugating cells. 

 In some genera of this family (such as Mesocarpus) the conjugating 



