162 Mr. G. H. K. Thwaites on the DiatomaceaG ; 



tion, maintain an independent existence (analogous to that of 

 the buds of the higher plant), retain all the functions necessary 

 for this independent life and for the propagation of the species, 

 and undergo no modification from their original form. The life 

 of an individual Diatomaceous plant we may therefore consider 

 to extend from the production of the sporangium to the period of 

 the conjugation of the numerous frustules which have originated 

 from it. 



Now, for the sake of comparison, let us analyse the higher 

 plant : we find it to consist but of a repetition of similar parts 

 and structure ; and by carrying the analysis still further we ascer- 

 tain the whole to be a modification of cellular structure, and that 

 this cellular structure is the product of a continued fissiparous 

 division commencing from the contents of the primordial cell — 

 the earliest condition of the embryo. In fact, that the entire 

 active vital part of the whole plant is but a diffusion, as it were, 

 of the contents (the endochrome) of the primordial cell. This 

 endochrome possesses an individuality — a character — which 

 though not appreciable by our senses, would be found, were it 

 in our power to analyse it, to be as defined as when, in its 

 further development, it has assumed all the marked peculiarities 

 of the species. The complicated development of the higher plant 

 may be said to be the expression of the quality of the endochrome 

 of its primordial cell, just as the simple development of the frus- 

 tule of the Diatomaceous plant expresses the quality of its endo- 

 chrome. If the foregoing is a correct view of the matter, it fol- 

 lows that the sporangium of the Diatomaceous plant is the ana- 

 logue of the primordial* cell of the flowering plant. 



We may now proceed more particularly to the subject of con- 

 jugation. In many of the Diatomacea it is seen that at a certain 

 period of the development of the species a union of the endo- 

 chromes of two distinct frustules seems necessary for the continued 

 existence of the species as well as for its reproduction. The 

 physiologist will endeavour to arrive at some probable explana- 

 tion of the reason why this mixture of endochromes is necessaiy, 

 and he will feel it difficult to come to any other conclusion than 

 this : namely, that in each of the conjugating endochromes an 

 essential element must to some extent, probably very trifling, be 

 wanting, whilst another essential element is in excess, and that a 

 mixture of such an endochrome with another similarly condi- 

 tioned, except that the quan^ties of such respective elements are 

 reversed, must take place in order to restore the equilibrium and 

 enable the species to continue its existence. The circumstance of 

 the mixed endochrome developing around itself a cell-wall pre- 



This must not be confounded with the " primordial utricle " of Mohl. 



I 



