SECT. I 



MORPHOLOGY 



91 



divides, the two daughter nuclei again divide, their successors repeat the process, 

 and so on, until at last thousands of nuclei are often formed. No cell division 

 accompanies these repeated nuclear divisions, but the nuclei lie scattered throughout 

 the peripheral, cytoplasmic lining of the embryo -sac. When the embryo -sac 

 ceases to enlarge, the nuclei surround themselves with connecting strands, which 

 then radiate from them in all directions (Fig. 96). Cell plates make their appear- 

 ance in these connecting strands, and fi'om them cell walls arise. In this manner 

 the peripheral protoplasm of the embryo-sac divides, simultaneously, into as many 

 cells as there are nuclei. All intermediate stages between simultaneous, multi- 

 cellular formation and successive cell division can be found in embryo-sacs. Where 

 the embryo-sac is small and of slow 



growth, successive cell division 

 takes place, so that multicellular 

 formation may be regarded as but 

 a shortened process of successive 

 cell division, induced by an ex- 

 tremely rapid increase in the size 

 of the cell. 



Free Cell Formation. — Cells 

 produced by this process differ con- 

 spicuously from those formed by 

 the usual mode of cell division, in 

 that the free nuclear division is 

 followed by the formation of cells, 

 which have no contact with each 

 other, and in the formation of which 

 the whole of the cytoplasm of the 

 mother cell is not used up. This 

 process can be seen in the develop- 

 ing embryo of the Gymnosperms, 

 in E'phcdra, for example, and also 

 in the formation of the spores of 

 the Ascomycetes. A single nucleus 

 is present to begin with in each ^^^' 

 ascus of the Ascomycetes. By 

 successive divisions eight nuclei 

 lying free in the cytoplasm are 







96. — Portion of the peripheral protoplasm of the 

 embryo- sac of Reseda odorata, showing tlie commence- 

 ment of multicellular formation. The process pro- 

 gresses from above downwards. From a fixed and 

 stained preparation, (x 240.) 



derived from this. A definite 



portion of cytoplasm around each of these nuclei becomes limited by a peri- 

 pheral layer which then forms a cell wall. Tims eight separate spores arise (cf. 

 Fig. 317). As the researches of Harper {^) have shown, the formation of the peri- 

 pheral layer proceeds from a centrosome-like mass of kinoplasm which formed a pole 

 of the spindle in the preceding nuclear division. The nucleus is drawn out towards 

 this mass of kinoplasm. From the latter kinoplasmic radiations proceed which 

 surround the spore as it becomes delimited, and finally fuse to form its peripheral 

 layer (Fig. 97). 



Cell-Budding. ^ — This is simply a special variety of ordinary cell division, in 

 which the cell is not divided in the middle, but, instead, pushes out a protuberance 

 which, by constriction, becomes separated from the mother cell. This mode of cell 

 multiplication is characteristic of the Yeast plant (Figs. 2, 331) ; and the spores, 

 known as conidia, which are produced by numerous Fungi, have a similar origin 

 (Fig. 343). i 



