164 Mr. G. H. K. Tliwaites 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 different in 
appearance and character ; one of them partaking rather of the 
character of Fuchsia fulgens 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 Fiatomacece, 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 ^Ir. 
Jenner pointed out to me some months ago, an apparent adum- 
bration of the sexes. The filaments of the genus Zygnema con- 
sist each of a single iwv of cells which correspond to the frustules 
of the Diatomaceae, 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 wdiich, wdth 
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. 
