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 Cucurbita, 
Helianlhus, Allium Cepa, &c.), between which lies the naked punctum vegetationis. 
But frequently this latter undergoes further growth before the seed is ripe, and 
produces additional foliar structures (as in Grasses, Fhaseolus, Faba, Quercus, A??i}'g- 
dalus, &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 Pai'thenogenesis^ . 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 coerulea, Scabiosa, 
Citrus), 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, Nothoscordwji fragrans, 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. 
Ccelebogyne 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 oosphere. 
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- 114- — Braun, Ueber Parthenogenesis 
bei Pflanzen, Berlin 1857; id., Ueber Polyembryonie und Keimung von Ccelebogyne, Berlin i860. — 
Braun und Hanstein, Die Parthenogenesis der Ccelebogyne 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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