IN THE VEGETABLE KINGDOM. 65 



of the fern ; one of them the micropyle leading through 

 the coats of the ovule, appears first when the prothallium 

 becomes internal, as in the Rhizocarp ; the other, that of the 

 style, being entirely extra-ovular, can, of course, have no 

 existence when the spores are deciduous, as in the Cryptoga- 

 mia, nor even when the ovule is uncovered, as in the Coniferse. 

 The true homology of the relation in which the various 

 intra-ovular structures of the Gymnospermese stand to those 

 of other Phanerogamia, is a point on which authors are 

 either not agreed, or else are so loose in their terminology 

 as to obscure their real meaning,* but the bearing of the 

 several envelopes to each other, and to the germ which 

 they enfold, may be represented to a certain extent, as fol- 

 lows, both in these groups and in the higher Cryptogamia, 

 without entering into the discussion : 



Angiosperms have three germ-envelopes viz., 

 Germen pervious through the style ; 

 Ovule-coats pervious by the micropyle ; 

 Embryo-sac imperforate. 



Gymnosperms have also three germ-envelopes, though 

 not all homologous with the foregoing, viz. : 



Ovule-coats pervious by the micropyle ; 

 Albuminous body (prothallium) perforated by the 

 pollen-tube ; 



* Thus the term endosperm is applied both to the tissue of the albumi- 

 nous body of the Coniferae, and to the cellular growth which takes place 

 within the embryo-sac in the course of development in many plants, and 

 which remains as a permanent constituent of the seed in Ranunculacese 

 and Nympheacese ; but, of course, the identification of these structures is 

 incompatible with a homology between the embryo-sac and the corpua- 

 culum or archegonium as much so, indeed, as the notion implied in the 

 old name of PistilUdium applied to the last mentioned organ. See the 

 article on Vegetable Reproduction in the Cyclopaedia of Anatomy and 

 Physiology. 



