12S 



MORPHOLOGY OP AXGIOSPERMS 



stage the chromatin thread splits longitudinally throughout its 

 entire length (Fig. 61, A, B). The double thread then seg- 

 ments transversely into the number of chromosomes characteris- 



D 



A 



Fig. 59. — Development of microspores in Heleocharis palvstris and Cars.r acuta. A-B, 

 Heleocharis. showing the single functional microspore and three disorganizing micro- 

 spores. x.380; after Strasblrgeb. 14 G-I, Carex: ' '. mother-cell; Z>, second division; 

 F, four nuclei, only three of winch are shown within the mother-cell * E and F 

 should be reversed i ; £, later stage than F; the nucleus of the functional micro- 

 spores is preparing for division; <?, tube nucleus, generative cell, and lower down 

 the nuclei of the three non-functional microspores; //. nearly ripe pollen grain; /, 

 irregular case in which the nuclei of the three non-functional microspores have 

 divided; x 900.— After J iel. 50 



tic of the gametophyte of a given species, each chromosome thus 

 being made up of two pieces (Fig. 53, C). According to several 

 investigator-, a second longitudinal splitting of the chromo- 

 somes may be seen during the anaphase of the first mitosis, so 

 that the two mitoses merely distribute the reduced number of 

 chromosomes which appear just after the segmentation of the 

 spirem. In the subsequent mitoses the spirem -segments into 

 chromosomes which afterward split longitudinally as in vege- 

 tative cells. 



It is in the divisions of the pollen mother-cell that the 

 problem of the reduction of chromosomes has been studied most 

 thoroughly: lmt while it is agreed that the reduced number 

 appears at the first mitosis, there is still some difference of 



