ANGIOSPERMS. 



393 



p,', 



fertilisation is effected in the ovules (Hofmeister). In the Orchideae pollination not 

 only stimulates the ovary to a vigorous and often long-continued growth, but makes 

 the ovules capable of fertilisation, and even in some cases is the cause of their first 

 appearance on the placentas which till then were sterile (Hildebrand). 



Results of fertilisation in the embryo-sac ; formation of the endosperm. The first 

 results of fertilisation are those which have just been described as appearing in the 

 egg-apparatus and in the oospore. The formation of the endosperm begins very often 

 before the division of the oospore, at the latest during its transformation into the 

 pro- embryo ; it begins in all cases with the division of the (secondary) nucleus of the 

 embryo-sac, and continues with the repeated division of the daughter-nuclei or 

 daughter-cells. Of this process of division 

 there are two modifications 1 . In a large 

 number of Dicotyledons the division of 

 the nucleus in the embryo-sac is com- 

 bined with cell-division ; the embryo-sac 

 after the division of its nucleus is divided 

 by a transverse wall into two cells, as for 

 example in Monotropa, Loranthaceae, 

 Orobanche, Labiatae and Campanulaceae, 

 and the further division of these cells gives 

 rise to the tissue of the endosperm, which 

 in this case often fills only certain parts 

 of the embryo-sac. In some cases the 

 embryo-sac also divides by a transverse 

 division into two daughter-cells, the 

 upper one of which contains the rudiment 

 of the embryo and produces a small 

 quantity of endosperm in a way which 

 will be described presently ; examples of 

 this proceeding are found according to 

 Hofmeister in Nymphaea, Nuphar, Cerato- 

 phyllum and Anthurium. The second 

 mode of formation of endosperm is by free 

 cell-formation and occurs in Monocoty- 

 ledons and most Dicotyledons. In this case 

 the embryos-sac is not at first divided into 

 compartments by cell-walls, but the nucleus 



divides and the two daughter-nuclei repeat the division, and the further continuance 

 of the process gives rise to a large number of free nuclei lying in the protoplasm 

 which lines the embryo-sac. These nuclei are almost always distributed as a simple 

 layer on the lateral walls of the embryo-sac, but a thicker layer of protoplasm is in 

 many cases collected at its two extremities about the oospore and about the antipodal 

 cells, and the nuclei then form several layers at the same points. Meanwhile the 

 embryo-sac has- increased considerably in size, and cell-formation does not begin till 



FIG. 323 



la tricolor. A longitudinal section of the ana- 



tropous ovule after fertilisation ; // the placenta, 7v cushion on the 

 raphe, a outer i inner integument, KK nucellus, / the pollen- 

 tube which has penetrated into the micropyle, e the embryo- sac 

 containing the embryo to the left and numerous free nuclei. B ind 

 C the convex apices of two embryo-sacs e, with the differentiating 

 pro-embryo eb attached to them, the suspensor in B being formed 

 of two cells. 



FIG. 324. Viola tricolor. Posterior portion of the embryo- 

 sac ; e its wall, S sap-cavity, K free nuclei imbedded in the 

 protoplasm fr. 



1 Strasburger, Zellbilding u. Zelltheilung, III ed. Jena, 1880. 



