OF THE DlAT031EiE. 71 



plants, often remain for a long period dormant, and are borne about by cur- 

 rents or become imbedded in the mud of the waters in which they have been 

 produced, until the circimistances necessary to their development concur to 

 call them into activity. At such times theii' sihcious epiderms open to per- 

 mit the escape of the contained endochrome, which is resolved into a myriad 

 of embryonic fnistules ; these either remain free or surround themselves Tvdth 

 mucus, fonning a pellicle or stratum, and in a definite but unascertained 

 period reach the mature form of the ordinary frustule," when their fiu'ther 

 growth appears almost entii^ely arrested by the production of the sihcious 

 coat, and when multiphcation by self-division provides for the continuation of 

 individual life. To continue the quotation, '' The size of the mature frustule 

 before self- division commences is, however, dependent upon the idiosjTicrasy 

 of the embryo, or upon the cii'cumstances in which its embryonic growth takes 

 place ; consequently a very conspicuous diversity in their relative magnitudes 

 may be usually noticed in any large aggregation of individuals, or in the same 

 species collected in different locahties." 



The behef that the contents of the sporangial frustules resolve themselves 

 into a ' brood' of Diatoms, having the same form and specific characters as 

 the original parent-cells. Prof. Smith establishes by the following observations 

 made by himself (Si/no2:)sis, vol. ii. p. xv) : — " In the gathering of Cocconenia 

 Cistula made in April 1852, which contained numerous instances of the con- 

 jugating process, I observed the frequent occiuTence of cysts enclosing minute 

 bodies variable in their number and size, and many of which had the outhne 

 and markings of the siUTounding forms and were obviously young frustules of 

 the Cocconema. It would appear that the production of the young frustules 

 is preceded by the separation and throwing off of the sihcious valves of the 

 sporangium and the constriction or enlargement of its primordial utricle, 

 according to the number of young frustules originating in its protoplasmic 

 contents. In this gathering, forms of eveiy size, intermediate between the 

 minutest fiiistule in the cyst and the ordinary frustules engaged in the 

 conjugating process, were easily to be detected ; and the conclusion was 

 inevitable, that the cysts and their contents were sporangia of the species 

 with which they were associated, and indicated the several stages of the 

 reproductive process." 



Again, in a gathering of Synedra radians, although not found at the 

 time in a congregating state, yet the appearance of the cysts and of their 

 contents was equally characteristic of the reproductive process. That such 

 a " cystoid condition is one stage in the normal development of its reproduc- 

 tion," a subsequent examination in a distant locahty satisfied him. 



The prosecution of this inquiiy into the final changes of the sporangial 

 fnistules is seriously impeded by the dissolution of the investing mucus and 

 the consequent dispersion of the reproductive bodies. 



Thirty-two species of the Diatomeae have been observed in the act of con- 

 jugation, belonging to the genera Epitliemia, Cocconeis, Cocconema, CymbeUa, 

 Cyclotella, GompTionema, Himantidium, Achnantlies, Rhahdonema, Melosira, 

 Navicida, Surirella, Amphora, Orthosira, Encyonema, Colletonema, and Schizo- 

 nema. On this paucity compared with the number of known genera. Prof. 

 Smith has the following explanatory remarks {Synops. ii. p. xi) : — "One reason 

 for the paucity of observations on this process in the Diatomeae is no doubt to 

 be foimd in the changes which usually take place in the condition of these 

 organisms at this period of their existence. During conjugation the progress 

 of self-division is arrested, the general mucous envelope or stratum produced 

 during seK-division is dissolved, and the conjugating pairs of frustules become 

 detached from the original mass ; they are thus more readily borne away and 



