4^6 PHANEROGAMS. 



GROUP IV. 



PHANEROGAMS. 



The Alternation of Generations in Phanerogams is concealed in the formation 

 of the Seed, which, at least in its earliest stage, consists of three parts: — (i) The 

 Tes/a, which is a part of the mother-plant; (2) The Endosperm^; and (3) The 

 Embryo, which is the product of the development of the fertilised oosphere. 



In Vascular Cryptogams we have already seen the sexual generation which 

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

 as an independent plant. In the Ferns, Equisetaceae, and Ophioglossaceas it grows 

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

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

 spore, the female prothallium still protruding, in the former group, 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 Coniferae this metamorphosis is carried one step further; the pro- 

 thallium ^, which is now known as the Endosperm, remains during its whole existence 

 enclosed in the macrospore or E?jibryo-sac ; it produces before fertilisation arche- 

 gonium-Hke structures, the ^ Corpuscula,' in which the oospheres arise. The pro- 

 cesses 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 Selagtnella. In this genus, besides the prothallium which pro- 

 duces 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 '\ If, therefore, the embryo-sac is the 



^ The only reason why the ripe seeds of many Dicotyledons do not contain any endosperm is 

 because it has already been absorbed and supplanted by the rapidly growing embryo before the seeds 

 become ripe ; while in others this absorption happens only on germination after the ripening of 

 the seeds, i. e. on the unfolding of the embryo ; more rarely the formation of endosperm is from 

 the first rudimentary. 



^ The analogy of the endosperm with the prothallium of the higher Cryptogams was first shown 

 by Hofmeister (Vergleich. Untersuch. 1851), [Germination, Development, and PVuctification 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 a rudiment of the true 

 prothallium. [According to Strasburger (Angiospermen und Gymnospermen, 1879) not only the 

 antipodal cells and the egg-apparatus, but also the endosperm of the embryo -sac of Angiosperms, 

 represent the prothallium (endosperm) of the Gymnosperms and of the Vascular Cryptogams. This 

 view leaves the ' endosperm ' of Selagtnella without any representative in other groups of plants. 

 Goebel has however expressed the opinion (Bot. Zeitg. 1880) that the endosperm of Selaginella 

 corresponds to the antipodal cells of Angiosperms.] 



