ANGIOSPERMS. 593 



already been mentioned when speaking of the endosperm. The external differ- 

 entiation sometimes goes no further than the rudiment of the root (radicle) at the 

 posterior end of the stem of the embryo, and the cotyledons {e.g. in Cucurbiia, 

 Helianlhus, Allium Cepa, &c.), between which lies the naked punclum vegetationis. 

 But frequently this latter undergoes further growth before the seed is ripe, and 

 produces additional foliar structures (as in Grasses, Phaseolus^ Faba^ Quercus, Amyg- 

 dalusj &c.), which are then included, in the ordinary nomenclature, under the term 

 Plumule^ but do not unfold until the germination of the seed. The systems of 

 tissue are usually sufficiently clearly differentiated as such at the period of maturity 

 of the seed ; but the different forms of permanent tissue do not become developed 

 till later, during germination. A striking exception to this advanced development 

 of the young plant within the ripening seed is afforded by parasites and ' saprophytes 

 destitute of chlorophyll, but especially by Orchideae. In them the embryo remains 

 until the seed is ripe as a small round body consisting sometimes of only a few 

 cells, without any external differentiation into stem, leaves, and root; this takes 

 place only after germination, and even then sometimes quite imperfectly. 



Polyembryony and Parthenogenesis^. In a few cases polyembryony, that is the 

 presence of more than one embryo in a single seed, has been found to occur in 

 Angiosperms, but it is brought about in a way which is very different from that 

 in which, as we have seen, it is caused in Gymnosperms. It was thought by 

 Hofmeister that, in the cases which he investigated {Funkia ccerulea, Scabiosa, 

 Cilrus), a number of oospheres were formed in the parietal protoplasm of the embryo- 

 sac, and that these were fertilised, but that of the large number of rudimentary 

 embryos thus formed, which is very considerable especially in Citrus, only a few 

 become fully developed. [This subject has been carefully investigated by Stras- 

 burger, and he has found that these embryos are not formed from oospheres, but 

 are developed as outgrowths from the cells of the nucellus which bound the embryo- 

 sac. In some cases {Funkia^ Nothoscordum /ragrans, Citrus) it appeared as if this 

 adventitious development of embryos were dependent upon the fertilisation of the 

 oosphere ; a development of an embryo from the oospore, in addition to the ad- 

 ventitious development of embryos from the nucellus, was only observed in Citrus. 



Coelebogyne ilicifolia has long been known as a plant with polyembryonic seeds, 

 and it has been observed that these fertile seeds are produced without pollination 

 of the female flower. It was concluded that this plant was an instance of partheno- 

 genesis, that is, of the development of an embryo from an unfertilised oospherfe. 

 Strasburger has found that its numerous embryos are developed adventitiously in the 

 manner described above : it is therefore not parthenogenetic. ] 



Development of the Seed and Fruit. While the endosperm and embryo are 

 becoming perfectly formed in the embryo-sac, growth proceeds not only in the 

 ovule but also in the wall of the ovary that encloses it. Since the testa is formed 



^ [Hofmeister, Die Lehre von der Pflanzenzelle, 1867, P- "4- — Braun, Ueber Parthenogenesis 

 bei Pflanzen, Berlin 1857; id., Ueber Polyembryonic und Keimung von Coelebogyne, Berlin i860. — 

 Braun und Hanstein, Die Parthenogenesis der Coelebogyne ilicifolia, in Hanstein's Botanische Abhand- 

 lungen, III, 1877.— Strasburger, Ueber Befruchtung und Zelltheilung, 1878. See also the section on 

 Parthenogenesis in Book III.] 



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