220 PHILOSOPHICAL NOTES ON 



hairs also confluent above and below (or on both sides of the 

 confluent sori) so as to form two foliaceous indusia, with the tips 

 of the hairs onlj free. By this we see that it is possible to 

 conceive every foliaceous indusium to have originated from a 

 confluence or fusion of simple hairs. 



The margin of the pinna of this Pteris aquUina is thickened 

 into a placenta, out of which sporangia (or fertile hairs) have 

 developed. If we now contract the pinna of this Pteris to its 

 simple mid vein, that is, abort the mesophyl, we shall have an 

 organ which does not essentially differ from an anther, ivith its 

 two loculiy like that of Magnolia,* except that the spores are 

 contained in sacs, while the pollen grains are free. Spores and 

 pollen grains are unicellular bodies, they germinate in a similar 

 manner, and have been compared to each other by Berkeley. 



Then in the fertile pinna of Blechnum boreale, Sw. (Fig. 71), 

 we have a single indusium on each margin of the pinna, which 

 arrangement, by contraction of the pinna, would give us the 

 rudiment of an anther ivith only one cell, the two opposite indusia 

 forming the two flaps of a one-celled anther. 



Asa Grray (" Structural Botany," p. 255) gives a diagram to 

 explain how pollen can be formed between the anterior and 

 posterior layers of the leaf-blade. Of course many things are 

 possible in nature, but as we have similar productions ready to our 

 hands in more ancient and less differentiated forms of plants, I 

 see no good reason why we should go out of our way to invent 

 new and more difficult modes of origin of pollen. 



The sporangia of ferns emerge from placentas situated on the 

 veins of the frond. Veins, according to my mode of thinking, are 

 homologous to branchlets, therefore sporongia are homologous to 

 buds, either branch-buds, or leaf -buds, or hair-buds, or any bud 

 the reader likes to think of. Whether we call them leaf -buds or 

 branch-buds is of little moment, there being no essential difference 

 between them. 



In many ferns the sori are naked, like clusters of buds, or 



ovules, without indusium, as in Polypodium. But in others they 



are covered by either a peltate indusium, as in Aspidium, or by 



a bract-like indusium, subtending the sorus, as in Nephrodium, 



Cystopteris, and others. In Asplenium Jilix fcemina, Bernh., the 



♦ Le Maout and Decaisne, " Syst. of Bot.," p. 193. 



