INTRODUCTION. XV 



Plate XLIX. 329/', and Plate LI. 331./. sp. ; Orthosira, Plate E. 

 337. III. IV. & V. ; and in Himantidiiim, Plate I). 280. IV. How 

 far this self-division may be carried in the sporangial frustules is 

 at present unknown ; it is probably of short duration, as we 

 rarely meet with any considerable number of frustules charac- 

 terized by the enlarged size of the sporangial form. In most 

 cases an arrest of growth, and consequently of self- division, 

 seems immediately to follow the complete formation of the 

 sporangia, and the reproductive body assumes the quiescent 

 character which belongs to the seed of the higher plant, its 

 vital function remaining dormant until circumstances favour its 

 further development, and the production of the young frustules, 

 of which it is the destined parent. 



The mode of this development is imperfectly understood, the 

 dispersion of the sporangial frustules by the dissolution of the 

 investing mucus usually removing the reproductive bodies from 

 the sphere of ready observation : cases, however, have fallen 

 under my notice, which seem to indicate that the further pro- 

 cess of reproduction consists in the resolution of the contents of 

 the sporangium into a " brood " of Diatoms having the same 

 form and specific characters as the original frustules which 

 originated the sporangia. In the gathering of Cocconema Cis- 

 tula made in April 1852, which contained numerous instances 

 of the conjugating process, I observed the frequent occurrence 

 of cysts enclosing minute bodies, variable in their number 

 and size, and many of which had the outline and markings 

 of the surrounding forms, and were obviously young frustules 

 of the Cocconema : these cysts and their contents are figured 

 in Plate C. 221. III. IV. & V. It would appear from these 

 figures, that the production of the young frustules is preceded 

 by the separation and throwing-ofF of the siliceous valves of the 

 sporangium, and the constriction or enlargement of its primor- 

 dial utricle, according to the number of young frustules ori- 

 ginating in its protoplasmic contents. In this gathering, forms 

 of every size intermediate between the minutest frustule in the 

 cyst and the ordinary frustules engaged in the conjugating 

 process (Plate C. 221. I.) were easily to be detected, and the 

 conclusion was inevitable, that the cysts and their contents were 



