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 v/hen the spores are deciduous, as in the Cryptoga- 

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

 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 albunii- 

 nous body of the Coniferse, 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 llanunculacete 

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

 incompatible with a homology between the embr^'O-sac and the corpus- 

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

 old name of PistilUdiii/tn applied to the last mentioned organ. See the 

 article on Vegetable Reproduction in the Cyclopaedia of Anatomy and 

 Physiology. 



