HOFMEISTER, ON THE HIGHER CRYPTOGAMIA. 337 



flattened stem the peculiar form of the wood and the 

 annual renewal of the bark by the vitality of a cambium 

 layer surrounding the wood. Since Von Mohl's disco- 

 veries the special attention of botanists has been almost 

 constantly directed to this interesting family. Alexander 

 Braun * pointed out the connexion between the \ or 

 arrangement of the fronds of young plants, and the two or 

 three-lobed form of the stem; he discovered the true 

 nature of the regular bifurcation of the roots, which had 

 been taken by earlier observers t for accidental lateral 

 ramification. He endeavoured to explain the remarkable 

 relation of the roots to the stem by the assumption that 

 " the roots in Isoetes, instead of breaking forth outwardly 

 from the vascular cylinder, penetrate, on the contrary, in 

 an inward direction." Mettenius,| about the same time, 

 gave a very accurate account of the structure of the ripe 

 spore, and suggested that the germination of Isoetes might 

 agree with that of Selaginella, as to which, at the same 

 time, he published the first correct microscopical obser- 

 vations. A year afterwards, Karl Muller gave an account 

 of the germination of Isoetes lacustris. || He describes the 

 large spore (of which he had only advanced specimens 

 before him) as a cellular sac, enclosed by an exosporium, 

 and in whose cavity the rudiment of the embryo appeared 

 in the form of a free cell, which was gradually transformed 

 into the cellular body. Mettenius forthwith corrected the 

 most essential errors of this account.^" He proved anew, 

 in a striking manner,** the similarity of the germination of 

 Isoetes and the other vascular cryptogams, by the discovery 

 of the formation of spermatozoa in the small spores, and 

 the description of the origin of the archegonia upon the 

 prothallium developed by the large spores. 



The following observations will afford some facts 

 supplementary to those noticed by Mettenius and Muller, 



* 'Flora,' 1847, p. 32. 



f Bischoff, 'Krypt. Gewaclise,' Niirnberg, 1828, p. 10. 



% 'Linnsea,' 1847, p. 269, on Azolla. 



1. c, p. 270, note. 



|| 'Bot. Zeit.,' 1848. 



% 'Bot. Zeit.,' ] 848,' p. 688. 



** 'Beitrage zur Botanik.,' Heft 1, Heidelberg, 1850. 



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