2 The llev. W. Smith on the Diatomacese. 



and the important questions involved in their nature and func- 

 tions demand from the physiologist. 



Without attempting authoritatively to deterniine these ques- 

 tions, 1 shall accompany the descriptions I shall give with such 

 notes on the structure, mode of growth, and general physiology 

 of these minute oi-ganisms, as have been suggested by the re- 

 peated and careful examinations to which I have submitted them, 

 and may seem to have a bearing on the discussions respecting 

 their nature, as belonging to the animal or vegetable kingdoms. 



This latter point is still a subject of controversy with the most 

 distinguished writers upon the subject, and it is therefore im- 

 portant that the facts noted by independent observers should be 

 carefully recorded, as it is only from the study and comparison 

 of these facts that a theoiy can be established which shall meet 

 the varied conditions in the life of these singular and beautiful 

 structures, and secure the general assent of naturalists familiar 

 with their forms. 



The discoveries of Mr. Thwaites respecting the conjugation of 

 the Diatomacece, as recorded in the ' Annals of Natural History ' 

 (vol. XX. pp. 9 & 343, also Second Series, vol. i. p. 16), seem to 

 place the vegetable nature of these forms almost beyond a doubt. 

 The process detailed and figured by Mr. Thwaites is perfectly 

 analogous to the formation of the sporangium in the Desmidiece 

 and many of the filamentous Algae, and may be considered as 

 essentially the same as that which takes place in the higher 

 ti'ibes of plants, the contents of the pollen-tube conjugating with 

 the contents of the ovule to form the embryo of the future seed. 



It is of importance that the facts noticed by ]\Ir. Thwaites 

 should be shown to be of general occurrence, and that their 

 existence be borne out by the concurrent experience of other 

 observers. I have great pleasure in confirming nearly all the 

 instances of conjugation in the Diatomacece mentioned by my 

 acute friend, and shall have the opportunity in the course of these 

 papers of recording several additional cases of the same process. 



It is well known to the student of the Diatomaceee, that in- 

 crease by fissiparous or self-division is uaiversal in the tribe. 

 This mode of growth — for such a method of increase is strictly 

 an extension of the individual and not a reproduction of the spe- 

 cies — has also an important bearing on the question of the ani- 

 mal or vegetable nature of these organisms. 



It seems to be a law of vegetable growth, that all increase in 

 the plant, from the multiplication of the separated cells of the 

 PalmellecE and Desmidiece, through the cells of the filamentous 

 Algse adherent only at their exti'emities, to the complex cellular 

 and vascular tissues of the higher tribes, shall take place by a 

 method analogous to fissiparous division, that is, shall com- 



