402 



FOURTH GROUP. SEED-PLANTS. 



wfi 



FIG. 333. Diagrammatic representation of the formation 

 of the primary root in Monocotyledons and its connection with 

 the stem ; i> suspensor, h hypophysis, iu TV boundary-line f root 

 and stem, -wh layer of the root-cap, d dermatogen, /* periblem, 

 pi plerome, ;' initial cells. From a drawing by Hanstein. 



The adventitious embryos thus formed continue to grow into the cavity of the 

 nucellus, pushing the embryo-sac before them, and assume altogether the habit of 



true embryos. A similar proceeding has 

 been observed in Nothoscordum fragrans. 

 In Citrus Aurantium the adventitious 

 embryos originate in single cells of the 

 nucellus lying either at the apex of the 

 embryo-sac or lower down, and may even 

 be separated from the embryo-sac by 

 several other cells. Coelebogyne ilicifolia 

 is interesting from the fact that it has 

 hitherto been adduced as an instance of 

 parthenogenesis in Phanerogams. Though 

 the female plant of this dioecious species is 

 the only one cultivated in Europe, it never- 

 theless often produces seeds capable of 

 germination, and these often have more than 

 one embryo. Here too there is a normal 

 egg-apparatus, but the oosphere cannot be fertilised as there is no pollen, and soon 

 perishes. This therefore is no case of parthenogenesis, which would only occur 

 if an embryo proceeded from the unfertilised oosphere, as in Char a crinita, (p. 64), 

 but embryos are formed, as in Funkia and Nothoscordum, as shoots from cells of 

 the nucellus. This is evidently a parallel case to that of the apogamous prothallia 

 of the Ferns which have been described above, in which the formation of an embryo 

 is replaced by asexual formation of a shoot. Whether the oosphere of Coelebogyne 

 is not still capable of fertilisation and development is not certainly known. 



Polyembryony may occur in Angiosperms in other ways than by the formation 

 of adventitious embryos. Two seeds are often found in the embryo- sac of some 

 Orchideae, as in Gymnadenia conopsea, the result probably of duplication of the 

 oosphere before fertilisation. Abnormal cases also are found of two nucelli in an 

 integument, each of which may produce an embryo. It was stated above that more 

 than one embryo-sac is formed in some ovules ; but as only one developes, no 

 occasion is given for polyembryony, at least in the cases as yet observed. 



Development of the seed and fruit. While the endosperm and embryo are being 

 formed in the embryo-sac, development takes place both in the ovule and in the wall 

 of the ovary which surrounds it. The seed-coat is formed from certain layers of 

 cells of the integuments or from the whole of their tissue, and its structure varies 

 greatly ; the ovule becomes the seed when the changes above detailed take place 

 within it as the result of the act of fertilisation. The wall of the ovary, the placentas, 

 and the dissepiments increase in volume and undergo manifold changes in external form 

 and still more in internal structure ; they and the seeds together form the fruit. The 

 altered wall of the ovary now bears the name of pericarp; if an outer layer is specially 

 differentiated in it, it is called the epicarp and the inner layer the endocarp; not 

 unfrequently there is a third layer lying between the two, the mesocarp. A series of 

 typical forms of fruit are distinguished according to the original form of the ovary 

 and the structure of its tissue in the ripe state, the nomenclature of which will be 



