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company ; otherwise why should they be formed with such 

 regularity ? They are not peculiar to one species, but are 

 found on the young fronds of all, as well on those from Cape 

 Horn and New Holland, as on the common species of our own 

 coasts. Similar fibres are found on the young parts of other 

 algse, especially of the Sporochnoidece and Dictyoteoe, in the 

 former of which they are evidently very essential organs ; and 

 I am of opinion that the monosiphonous ramuli oi Polysiphonia 

 byssoides and its allies, and of all the Dasyce, are organs of a 

 similar nature, but are in higher development in those plants 

 than in the majority of the Polysiphonice. Imperfect as they 

 seem to be, I am inclined to regard them as leaves, or the ana- 

 logues of those organs. 



" The only argument that occurs to me why we should not 

 regard these fibres as acrogenous leaves is founded on their 

 minute size and imperfect development. But this can be no 

 valid objection to their analogical character. Even among 

 perfect plants, such as Exogens, we often find the leaf reduced 

 to a minute scale, while its place is supplied by a peculiar fron- 

 dose development of stem, as in the Cacti, and, in a still more 

 striking manner, in the euphorbiaceous genus Xylophylla. 

 In this latter the small branches are flattened and green, like 

 leaves, while the true leaves are reduced almost to simple 

 fibres, and are only found on the young branches. Compared 

 with the organization of Xylophylla, such leaves are incompa- 

 rably less perfect than the fibres oiPolysiphonitje, as compared 

 with the organization of that genus. Imperfection of develop- 

 ment is, therefore, no valid objection to the analogy between 

 the apical fibres and acrogenous leaves ; and if this analogy be 

 admitted, we establish the first step in our argument. 



" We have in the next place to determine the morphological 

 relation of the ceramiduim or case in which the tuft of spores 

 is contained. This spore-case is, in all the Rhodomelece and 

 ChondriecB, simply a truncated branch of the frond ; a branch 

 diverted from its normal character, and changed into an ovate 



