4■^ 



PHANEROGAMS. 



In Vascular Cryptogams we have already seen the sexual generation which 

 results directly from the spore or prothallium losing more and more of its character 

 of an independent plant. In the Ferns, Equisetacese, and Ophioglossacese it grows 

 independently of the spore, often for a considerable period ; in the Rhizocarpese and 

 Lycopodiace^, where male and female spores are formed, it arises in the interior of 

 the spore, the female prothallium still protruding in the former out of the cavity of 

 the macrospore, but remaining united with it ; while in Isoetes it fills up the interior 

 of the macrospore as a mass of tissue which only bursts the cell-wall of the spore in 

 order to render the archegonia accessible to the antherozoids. In the Cycadeae and 

 Conifers) this metamorphosis is carried one step further; the prothallium \ which is 

 now known as the Endosperm, remains during its whole existence enclosed in the 

 macrospore or Embryo-sac ; it produces before fertilisation archegonium-like struc- 

 tures, the ' Corpuscula,' in which the Germinal or Embryonic Vesicles arise. The 

 processes which take place in the embryo-sac of Monocotyledons and Dicotyledons 

 appear somewhat different, and bear a greater resemblance to what takes place in 

 the macrospore of Selaginella. In this genus, besides the prothallium which produces 

 the archegonia, there arises subsequently, by free cell-formation, another tissue which 

 fills up the rest of the space of the macrospore ; to this tissue the endosperm of 

 Monocotyledons and Dicotyledons, which is formed by free cell-formation only 

 after fertilisation, appears to correspond ; the prothallium of Selaginella does not 

 appear to have anything to correspond to it in Angiosperms, the embryonic cells 

 or ' Germinal Vesicles' arising immediately from the protoplasm of the embryo- 

 sad If, therefore, the embryo-sac is the representative of the macrospore, that 

 part of the ovule in which the embryo-sac arises (the nucleus) must be considered 

 the equivalent of the macrosporangium. But, as in the formation of the seeds of 

 IMonocotyledons and Dicotyledons, certain processes of development (the formation 

 of the archegonia or 'corpuscula'), being no longer necessary, are suppressed, and 

 the embryonic vesicle is produced immediately from the embryo-sac as the analogue 

 of the macrospore, so also the production of the embryo-sac immediately from the 

 tissue of the nucleus of the ovule is more direct. Its production is due to the 

 simple increase in size of an inner cell of the nucleus which here replaces the 

 sporangium. But -vhile even in the most highly developed Cryptogams the macro- 

 spore still becomes detached from the tissue of the mother-plant, and the full 

 development of the prothallium takes place only after the dissemination of the 

 spores, so that the embryo always arises in structures distinct from those of the 

 mother-plant, the embryo-sac (or macrospore) of all Phanerogams remains, on 

 the contrary, enclosed in the ovule, the endosperm in the embryo-sac, and the 

 embryo in the endosperm. In this manner arises that structure peculiar to Phane- 



* The analogy of the endosperm \\-ith the prothallivun of the higher Cryptogams was first shown 

 by Hofmeister (Vergleich. Untersuch. 1851), [Germination, Development, and Fructification of the 

 Higher Cryptogamia, Ray Soc. 1862, p. 438.] 



^ Compare Pfeffer in Hanstein's Botanical Dissertations, Heft IV, p. 24. The * Antipodal 

 Cells ' in the embryo-sac of Angiosperms may probably be considered as the last occasional occur- 

 rence of the rudiment of the true prothallium, and the occasional filamentary apparatus of the 

 embiyonic vesicles as the last rudiment of the canal cell. 



