308 Beer. Development of the pollen grain and anther of some Onagraceae. 



"rodlet" layor wliicli is iiiterniptod over tlie apices of the 

 intei'stitial bodies. 



During the hiter growlh vi thu pollcii yraiii tlie secondary 

 thickening hiyer has not increased in tliickncss but has, on the 

 contrary, become stretched and verv miich thinner than it was 

 at an earUer stage. (In Fig. 50 the secondary thickening layer 

 has beon drawn too tliick.) 



In Ooiutlicra lo)iyiflora all the pollen grains do not reach 

 maturity but a large proportion of them become arrested in their 

 development. They all grow to about 90 fi in diameter, when 

 their jirotoplast ha.s become reduced to a hollow shell, but after 

 that many of them are unable to continue their development 

 owing, no doubt, to the tapetal substance being insufficient for 

 the requirements of all the pollen grains. 



I have not given any special attention to the germination 

 of the jjollen grain but I may mention that the intine of 

 Epilohhim tetragonum which gives the reactions of both cellu- 

 lose and pectose, gi^ows out into a tube which is often branched 

 at its free end (Fig. 20). 



The mature pollen grains of Oenothera are bound together 

 in long strings by bundles of "fibrils" which he between and 

 round them. These tibrils are developed from the mucilage 

 which, on an earlier page, we saw was derived from the disinte- 

 gration of the special-mother-cell walls. These „fibrils" have 

 entirely lost all affinity for callose dyes and have become very 

 resistant to solvents. Their properties in many respects resemble 

 those of cuticularised structures. 



In the species of Epdohium short bands of the cuticularised 

 mucilage bind together the pollen grains which, consequently, 

 leave the anther in tetrads. 



Summary aiid couclusioiis. 



1. In the earliest stages of anther development all the cell- 

 membranes contain both cellulose and pectose. The walls of 

 the sporogenous cells, however, contain less cellulose than the 

 other membranes of the anther. In older anthers the sporo- 

 genous cell-membranes give the reactions of a j)ectic substance 

 alone. 



2. The pollen-mother-ceU wall consists of pm-e callose. This 

 substance is formed directly as such by the protoplast and there 

 can be no possibility, in the present case, of a transformation of 

 cellulose into callose. 



3. In the lirst and second divisions of the pollen-mother- 

 cell seven chromosomes occur whilst in the somatic divisions 

 fourteen is the apjDroximate number of chromosomes. The 

 presence of two nuclei, one large and the other very small, in 

 some quite young pollen grains suggests the occirrrence of irre- 



