104 



THE PLANT CELL, 



There are possibly other factors, especially in connection with 

 the increase in mass of the chromatin, which must require a supply 

 of phosphorus-containing food material, but these cannot be gone 

 into fully, the chemistry of the process being somewhat obscure. 



Microscopically, the first change to be noticed in the nucleus 

 is the increased capacity which this structure shows in the 

 taking up of such stains as haematoxylin or safranin. In this 

 respect it is the chromatin-granules which show this increased 

 staining capacity, the nucleoli not showing much difference at 

 first (early prophase). Next, the ehromatin-granules become 

 more regularly arranged upon the linin network, and soon the 

 appearance is presented of a definite chain of grannies set at 

 equal or nearly equal intervals apart upon a continuous coiled 

 thread of linin (see Fig. 76, 2). At a slightly later stage, careful 

 observation has shown that each chromatin-granule becomes 

 divided into two, so that there are then two parallel rows 

 of granules arranged regularly upon two threads of linin, the 

 latter structure also having undergone a similar fission to the 

 granules.* During this process, the chromatin-granules have 

 increased in size, and approached one another, so that, finally, 

 there seem to be two parallel threads coiled with the limits 

 of the nuclear membrane (see Fig. 76, 2, and Fig. 77, 3). A 

 good resolving power of the microscope is necessary to make out 

 the dual nature of the chromatin band. In the endosperm of 

 Fritillaria, and root-tip of Hyacinthus, during mitoses, it is, how- 

 ever, fairly obvious. 



These changes complete the early prophase, and the coiled 

 chromatin-band is now known as the spireme or skein (see 

 Fig. 77, 4). Traces of the nueleoli may still be seen at this 

 stage, but the nuclear membrane has already become indistinct. 



The phenomenon now occurs of the breaking up of the 

 spireme into a number of equal lengths of chromatin, known as 

 the primary chromosomes : this is effected by either mechanical 

 rupture or chemical absorption occurring in the linin-thread at 

 several equidistant points (see Fig. 78, 5). The number of the 

 primary chromosomes varies in diiferent plants, and may be as 

 many as twenty-four {Lilium}; and it is obvious that each 

 primary chromosome is a double structure. 



* This was definitely shown to occur in HeUehorus foetidxts, by Mottier, 

 and it can be observed in most cases. 



