26 



BOTANY 



PART I 



protoplasmic mass with many nuclei ; this is not divided into 

 chambers by cell walls. 



2. Deviations from typical Cell Division. The main deviations 

 from typical cell division which are found here and there in the 

 vegetable kingdom are MULTiCELLULAR FORMATION, CELL-BUDDING, and 



FREE CELL FORMATION. 



(a) Free Nuclear Division and Multicellular Formation. The nuclear division 

 in the multinucleate cells of the Thallophy tes may serve as an example of free nuclear 

 division, that is, of nuclear division unaccompanied by cell division. In plants 

 with typical uninucleate cells, examples of free nuclear division also occur. This 



method of development is especially 

 instructive in the embryo-sac of 

 Phanerogams, a cell, often of re- 

 markable size and rapid growth, 

 in which the future embryo is 

 developed. The nucleus of the 

 rapidly growing embryo-sac 

 divides, the two daughter nuclei 

 again divide, their successorsrepeat 

 the process, and so on, until at 

 last thousands of nuclei are often 

 formed. No cell division accom- 

 panies these repeated nuclear divi- 

 sions, but the nuclei lie scattered 

 throughout the peripheral cyto- 

 plasmic lining of the embryo-sac. 

 When the embryo-sac ceases to en- 

 large, the nuclei surround them- 

 selves with connecting strands, 

 which then radiate from them in 

 all directions (Fig. 19). Cell plates 

 make their appearance in these 

 connecting strands, and from them 

 FIG. 19. Portion of the peripheral protoplasm of the ce ^ wa n s arise. In this manner 

 embryo-sac of Reseda odorata. showing the commence- ,, r i f 4-v, Q 



ment of multicellular formation. This progresses the **&** protoplasm of the 

 from above downwards. From a fixed and stained embryo - sac divides simultane- 

 preparation. (x 240. After STRASBTJRGER.) ously into as many cells as there 



are nuclei. All intermediate stages 



between simultaneous multicellular 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 multicelluiar formation may be 

 regarded as but a shortened process of successive cell division, induced by an 

 extremely rapid increase in the size of the cell. 



(b) 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 (Fig. 20) ; the spores, known as 

 conidia and basidiospores, which are produced by numerous Fungi, have a similar 

 origin (Fig. 398). 



(c) Free Cell Formation. Cells produced by this process differ conspicuously 



