INTRODUCTION. 



demonstrate satisfactorily, unless a species of Conferva with 

 a coloured integument should be detected, or some means 

 can be devised for permanently colouring the filaments with- 

 out impairing their growth. Then indeed the question might 

 be determined ; at present I can merely show the probability 

 that the cell in cylindrical species of Desmidiese agrees with 

 the joint in a Zygnema or Tyndaridea ; since whenever the 

 covering is colourless and free from markings not the slightest 

 difference can be perceived. This is the case in a few species 

 of Penium ; and hence Penium Brebissonii is by some authors 

 placed in the Palmelleae. In Penium margaritaceum and 

 Penium Cylindrus the integument is coloured, and we are 

 enabled, by means of the paler appearance of the newly- 

 formed portions, to satisfy ourselves that in these also each 

 half of the original cell is acquiring during the division a 

 new partner. In Didymoprium the same fact is rendered 

 apparent, because the suture passes between minute teeth ; 

 these teeth recede from each other, and the new teeth which 

 appear between them show the place where the separation of 

 the joint has occurred. 



The spontaneous division of the frond is included by some 

 writers amongst the modes of reproduction ; but this is not 

 strictly correct, for it is rather the manner in which the in- 

 dividual plant grows, since all the cells arrive at maturity 

 nearly at the same period and terminate their existence about 

 the same time. 



The Desmidiese are most probably reproduced only in two 

 modes ; one by the escape of the granular contents of the 

 mature frond, and the other by the formation of sporangia, 

 the result of the coupling of the cells. 



When the cells approach maturity, molecular movements 

 may be at times noticed in their contents, precisely similar 

 to what has been described by Agardh and others as oc- 

 curring in the Confervae. This movement has been aptly 

 termed a swarming. It has been seen by numerous ob- 

 servers, — in this country by Messrs. Dalrymple, Jenner, 

 Thwaites, Sidebotham, Dr. Dickie, and others. The cause 



