90 



BOTANY 



TAUT I 



the multinuclear cells of the Thallophytes may serve as an example of free nuclear 

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

 with typical uninuclear cells, examples'of free nuclear division also occur ; although. 

 in that case, the nuclear division is customarily followed by cell division. While 

 the nuclei increase in number by repeated division, this process is not accompanied 

 by a corresponding cell division. When, however, the number of nuclei is com- 

 pleted, the cytoplasm between the nuclei divides simultaneously into as many 

 portions as there are nuclei. In this process we have an example of multicellular 

 formation. This method of development is especially instructive in the embryo- 

 sac of Phanerogams, a cell, 

 often of remarkable H/C 

 and rapid growth, in which 

 the future embryo is de- 

 veloped. The nucleus of 

 the rapidly growing em- 

 bryo-sac divides, the two 

 daughter nuclei again 

 divide, their successors 

 repeat the process, and so 

 on, until at last thousands 

 of nuclei are often formed. 

 Xo cell division accom- 

 panies these repeated nuc- 

 lear divisions, but the 

 nuclei lie scattered 

 throughout the peripheral, 

 cytoplasmic lining of the 

 embryo-sac. When the 

 embryo-sac ceases to en- 

 large, the nuclei surround 

 themselves with connect- 

 ing strands, which then 

 radiate from them in all 



round the mass of cytoplasm ; in C this process is com- directions ( Fi g- ^5). Cell 



plete, but the limiting layer produced by the fusion of radia- plates make their appear- 



tions of the fibrillar plasma is still connected with the polar ance in these connecting 



side of the nucleus ; in D this kinoplasniatie connection strands and from them 

 between nucleus and limiting layer has disappeared ; *, n 11 ; T~ <-\ 



,'r. cell walls arise. In tins 

 nuclear network ; n, nucleolus. (After HARPER, x 1500.) 



manner the peripheral 



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

 are nuclei. Various intermediate stages between simultaneous, multibellular forma- 

 tion and successive cell division can often be observed in an embryo-sac. Win-re 

 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 extremely, rapid increase in the size of 

 the cell. 



Free Cell Formation. Cells produced by this process differ conspicuously 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 developing embryo of the Gymnosperms, in 



Fio. 96. Successive stages of the delimitation of a spore in the 

 ascus of Erysiphe eommunis. A, Before delimitation has 

 begun ; the tibrillar plasma (kp) radiates into the cytoplasm 

 around; in B the fibrillar plasma has commenced to grow 



