164 Mr. G. H. K. Thwaites on the Diatomacese ; 



idea of such an explanation of the phsenomena, for in one ex- 

 ample of the hybrid Fuchsia seedlings the singular circumstance 

 occurred of one seed producing two plants extremely diiFerent in 

 appearance and character ; one of them partaking rather of the 

 character of Fuchsia fulg ens and the other of Fuchsia coccinea. It 

 cannot be doubted that these very dissimilar structures were the 

 produce of one seed, since they were closely coherent, below the 

 two pairs of cotyledon leaves, into a single cylindrical stem, so 

 that they had subsequently the appearance of being branches of 

 one trunk. The plant was unfortunately, before flowering, killed 

 by an unexpected severe frost, but not before its peculiarity had 

 been observed by many persons besides the writer. In the case 

 just cited the idea of a modification of structure caused by mere 

 circumstance of situation in the early stage of growth is quite 

 untenable ; for were such the case, it is clear there could not have 

 been the great dissimilarity which presented itself in these twin- 

 plants — the produce of a single seed. 



The following explanation of the phsenomena of hybridization 

 appears to the author to be most probably the correct one ; 

 namely, that the hybrid embryo consists, like an ordinary em- 

 bryo, of a mixture of two endochromes — one derived from the 

 pollen-grain and the other from the ovule; and that the peculiar 

 character of each hybrid individual is due to the preponderance 

 of one or other of these endochromes. This view of the matter 

 seems to remove much of the mystery which at present surrounds 

 this subject. 



Returning again to the consideration of the lower plants. It 

 is true that in the Diatomacece, as far as has been yet observed, 

 there is no appearance of a difference of sex — there is nothing to 

 indicate a diversity in the character of two conjugating frustules. 

 In an allied family, however, the Conjugatece, there is, as Mr. 

 Jenner pointed out to me some months ago, an apparent adum- 

 bration of the sexes. The filaments of the genus Zijgnema con- 

 sist each of a single row of cells which correspond to the frustules 

 of the Diatomacece, like them undergoing fissiparous division and 

 becoming conjugated. Conjugation of the cells of Zygnema 

 however takes place by the endochrome of one cell finding its 

 way into another cell and there mixing with its endochrome ; so 

 that the sporangium is formed in one of the cells instead of out- 

 side both cells, as in the Diatomacece. In several species of 

 Zygnema some of the filaments consist of cells, all of which, with 

 the rarest exception, after conjugation contain the sporangia, 

 whilst all the cells of other filaments of the same plant are seen 

 to have entirely parted with their endochrome. This has much 

 of the aspect of sexuality, and the sight of the conjugated fila- 

 ments suggests at once this idea to the mind. 



