BRITISH DESMID1AOE.E. 



itself at one pole and then swung over and attached 

 itself by the other pole, thus shifting its position by a 

 distance equal to its own length. This is an exhibition 

 of periodical polarity brought about by an alternation 

 of positive and negative heliotropism or geotropism, 

 or by a combination of both. Stahl's experiment was 

 performed by placing the Desmids in glass tubes and 

 altering the direction of the light by reflection. The 

 longer axes of the cells placed themselves parallel to 

 the incident rays, first one pole presenting itself 

 towards the light and then the other. The reversal of 

 position, which was a movement of the cell through an 

 angle of 180, occupied from six to thirty-five minutes 

 according to the temperature ; the change of position 

 occurring more rapidly as the temperature increased, 

 taking from six to eight minutes at a temperature of 

 33 C. In Peniam curhim the polarity is constant, the 

 young semicells always turning towards the light. 

 Goebel found that Micrasterias rohihi arranged itself 

 so as to place its plate-like chloroplasts at right 

 angles to the incident rays of light. Warming states 

 that the movements are due to the protrusion of a 

 mucilaginous stalk, and that they are partly dependent 

 upon, and partly independent of heliotropism and 

 geotropism. 



The VEGETATIVE REPRODUCTION of Desmids takes place 

 by the process of cell-division, but this presents a 

 rather curious appearance, reminding one of gemina- 

 tion, in those forms which possess a deep constriction. 

 The first stage in this cell-division is an elongation of 



o 



the isthmus causing the semicells to move slightly 

 apart. The elongated isthmus increases in size, 

 becomes somewhat swollen and turgid, and soon shows 

 signs of a constriction. The constriction deepens, and 

 when complete the two young semicells remain 

 attached by their apices. The new semicells are at 

 first much more delicate than the old ones, but they 

 gradually increase in size until they ultimately attain 

 the form and strength of the mature halves, when in 



