PROTOCOCCUS ; CYANOPHYCE.E 547 



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

 changes occurs which 1ms 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 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 'conjugation' of 

 two cells occurs, as in Pal/moglosa ; 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 probably be found to present themselves before and after the 

 performance of that act. 1 



The Cyanophycese or Phycochromacese constitute another group 

 of lowly forms of vegetable life, distinguished by their blue-green 

 colour, differing from the Protococcaceae in not containing true 

 chlorophyll grains, the cell-sap being, on the other hand, coloured by 

 a soluble blue-green pigment known as ' phycocyanin.' They live 

 either isolated, or a number congregated together and enclosed in a 

 more or less dense colourless jelly. They multiply by binary 

 division, and do not in any case produce zobspores. To the lowest 

 family of this group, which strongly resemble the Protococcacere, 

 except in the colour of the cells, the Chroococcacece, belong the genera 

 Chroococcus, Glosocapsa. Aphanocapsa, Jferismopedia, and many 

 others, the life-history of which is but very imperfectly known. 



The OscittatoriacecB constitute a family of Cyanophycese of great 

 interest to the microscopist, on account both of the extreme sim- 

 plicity of their structure and of the peculiar animal-like movements 

 which they exhibit. They 'consist of fine, usually microscopic 

 threads, containing a blue-green endochrome, sometimes replaced by 

 a red or violet, and occur singly or in thick strata in fresh running 

 or more abundantly in stagnant water. The threads are unbranched 

 and iisually straight, and either each separate thread or a number 

 together are, in most of the genera, enclosed in a gelatinous sheath. 

 Some illustrations of these are seen on Plate VII. The contents of 

 the sheaths are imperfectly divided into cells by transverse divi- 

 sion ; small pieces of the threads, consisting of a few cells, occasion- 

 ally break off, round themselves off at, both ends, move about with a 

 slow undulating motion, and finally develop into new threads ; these 

 portions are known as hormoyones. The most abundant genus. O.sv/7- 

 latoria, has been si > named from the peculiar < iscillatingor waving motion 

 with which the threads are endowed. This consists of a creeping 

 motion in the direction of the length of the thread, now backwards, 

 now forwards, accompanied by a curvature of the thread and rotation 

 round its own axis. The cause of this motion is still a matter of 



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 led him to adopt a view of the relationship of the ' still ' and ' motile ' 

 forms which is in essential accordance with his own. 



N X '2 



