THE LIFE-HISTORY OF A FERN 



[CH. 



to that seen in Vascular Plants at large, the substantive Plant being in all 

 cases the diploid sporophyte. 



The nuclear details of the normal chromosome-cycle have been observed 

 for the Male Shield Fern {Dryopteris) by Yamanouchi, and delineated as 

 shown in Figs. 29, 30. In the somatic divisions of the sporophyte the diploid 

 number of 128 chromosomes appears (Fig. 29, e): and that number is main- 

 tained in all the divisions leading up to the definite spore-mother-cells. 

 Fig. 29, a shows a section through a nucleus of a somatic cell, in the condi- 

 tion where the spirem has broken up into the constituent chromosomes. These 

 assemble later at the equator of the nuclear spindle {b) and divide longi- 



.^K<-T-/<}iW-5jf:3'^-/f.,.. 



-^p^^mC:^ i^^^^SSV- - 





f 



Fig. 2y. \egctalive mitosis in sporogenous cell of Dryopteris, after 

 Yamanouchi. a = spirem segmented into chromosomes; (5 = metaphase 

 showing equatorial plate; (- = anaphase, two sets of daughter chromosomes 

 separated; (/=late anaphase; f= polar view of stage seen in d, showing 

 128 chromosomes, which is the diploid number; /= daughter-nuclei after 

 formation of membrane ; the cell-plate has appeared equatorially between 

 the two nuclei. 



tudinally, the half-chromosomes separating (c) and collecting in the anaphase 

 at its two poles {d). Seen from the pole, in this condition, it is possible to 

 count the chromosomes, and their diploid number is seen to be 128 {e). 

 They then coalesce to form the new nuclei (/), while the equatorial plate 

 constitutes the partition wall between the cells. 



Similar observations of the divisions leading to the formation of the 

 spore-tetrad are illustrated in Fig. 30, a — k. The nucleus is at first clearly 

 delimited, showing a chromatic reticulum, with a nucleolus {a). The nucleus 

 enlarging enters the condition called "synapsis," from which it gradually 



