302 "Reor, Development of tlie pDllen grain aml aiither of some Onagraceae. 



The interstitial bodies aro sliiu off from tlic cavity of the 

 ])(»lU'ii ü;rain by a avoU developed closing-disc wliich consLsts of 

 two parts ; an oiiter denso and honiogeneoiis layer and an inner 

 less donse, stratitied laniina Avliicli is cap-shapod in form with 

 the eoneavity directed towards the cell cavity. Lying on the 

 onter himina of the closing-disc is an aggregation of granulär 

 material which extends some little way up the sides of the 

 interstitial body. The cavity of the interstitial body is no longer 

 occupied by tiie mucilaginous deejily staining substance which 

 iilled it at an earlier period, but it now contains a watery fluid 

 which does not readily stain. I believe that the mucilaginous 

 material of the young interstitial body has been, to a great 

 extent, used n\) in forming the closing-disc and that the granu- 

 lär substance which lies u])on the outer portion of the diso is a 

 remnant of the stainable material. (See Fig. 23.) 



In pollen grains which measure between 85 // and 95 // in 

 both 0. hiennis and 0. hngiflora the protoplast has become re- 

 duced to a liollow sphere or vesicle which has ex])anded again 

 until it is nearly or quite in contact with the cell-wall (Fig. 25j. 

 At one point upon the protoplasmic vesicle a flattened, rather 

 dense nucleus can be seen which encloses a nucleolus. A little 

 flnely granulär cytoplasm surrounds the nucleus but in its other 

 parts the protoplast appears to be reduced to a plasmoderma 

 (Hautschicht) which surrounds the enormous central vacuole. 

 Very soon the nucleus enlarges, becomes rounder and less dense 

 and passes into the prophases of mitotic division (Fig. 26). 



I liave not followed the details of this division which leads 

 to the formation of two distinct cells within the pollen grain : 

 the large vegetative cell and the small generative cell. The 

 latter is limited by a well marked plasmoderma (Hautschicht) 

 (Fig. 27). 



The tapetum now breaks down and its contents clearly 

 furnish the material for the renewed growth of the pollen- 

 protoplasts. 



In Order to understand the nature of this material it is 

 necessary to consider the changes which take place in the tape- 

 tum during its earlier development. 



In the very young anthers, before the füll number of pri- 

 mary sporogenous cells is established, the tapetal cells contain a 

 not very dense cytoplasm which. encloses a single nucleus. 



This nucleus, besides small, scattered chromatin granules, 

 contains one to four nucleoli. 



The nuclear membrane colours deeply with iron-haematoxylin 

 or with methylene blne-fuchsin mixture. Very rapidly the cyto- 

 plasm increases in density and the originaUy single nucleus 

 divides into several, as many as eight nuclei being not uncommonly 

 met with in a cell (Fig. 30 and 31). 



Until about the pollen - mother-cell stage the tapetal nuclei 

 multiply exclusively by mitotic division but at the mother-cell 

 stage nuclear iigures occur which are strongly suggestive of 



