AZOLLA AND SALVINIA. 



53/ 



called, with which in external appearance and hypothetical 

 capabilities of forming new plants they have something in 

 common. 



On the remarkable difference of the yellow sac. etc, in 

 Azolla being developed within the nucleus, to the exclusion 

 of the growths roun^its base, while in Salvinia each corres- 

 ponding sac is developed within a growth or protuberance 

 from the surface of the nucleus, I have nothing to offer. 

 Neither have I any thing to say in explanation of the pedi- 

 celled, mass-containing secondary capsules of Azolla being 

 developments of the basilar protuberances, to the exclusion 

 of the nucleus itself. It is a remarkable fact, however, that 

 in Musci and the vaginulated Hepaticae, the ovulum undergoes 

 no change except in situation, for it forms the extreme tip 

 or point of the mature seta. In Azolla something of the same 

 kind occurs, but in a limited manner, an opposite direction 

 and without change of situation ; for the nucleus, the part 

 first formed, may be found unchanged in the mature capsule. 

 And we are not in want of instances in which that part of a 

 phanerogamous ovulum, which is first formed and which is a 

 direct extension of the surface from which it grows, remains 

 equally unchanged during the development of the seed. 



The first general remark I have to make regards the simi- 

 larity of the organs in their younger stages to that form of 

 the ovulum of phanerogamous plants, in which the original 

 direction of development is preserved, and which are now 

 generally known by the term antitropous, or more correctly 

 atropous.* And though this simpler form of ovulum is not 

 always peculiar to particular families and not invariably even 



* Although the difference between the development of the vegetable 

 carpel leaf and vegetable ovulum is in general sufficiently apparent, an 

 exception has appeared to me to be presented by Naias, in which the 

 future pistillum seems to be derived from an annular growth round a 

 central body, which subsequently becomes the ovulum!! 



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