240 On Azolla and Salvinia, 



masses properly so 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 round 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 Hepaticse, the ovulum under- 

 goes 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 

 similarity of the organs in their younger stages to that form 

 of the ovulum of phanerogamous plants, in which the ori- 

 ginal 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 



* Although the difference hetween the development of the vegetable carpel leaf 

 and vegetable ovulum is in general sufficiently apparent, an exception has appear- 

 ed to me to be presented by Naias, in which the future pistillum seems to be de- 

 rived from an annular growth round a central body, which subsequently becomes the 

 ovulum ! I 



