iM)(\ l>t'('r. Development of the pullen giiiiii iiiul iintlicr of somo Ona,<;rnceae. 



walls become verv tliiii and at the liiiir wlirii the tapctnin 

 tlisintegrates tlicy Ijoconic so attciniatcd that at some spot-s tliey 

 are ajiparontly iiiti'iTupUul alto^dlici'. Ii i.s obvious therefore 

 that the tajK^tal walls offor no ^rcat liiiidcrance to tlio passa<2;<^ 

 of the cell-eoiitenls. It is iiiore tlil't'iciili. however. lo understand 

 how the tapetal siibstance passes tlii<iui!,li llic thi(d<, cuticuhirised 

 polk^n-\vall to reacdi tlie protoplast. It must evidently do so in 

 a State of sohlt ion l)ut how the oomplex iiiaterial of the tapetal 

 cells is brought iiito sohition can at present only he guessed at. 

 Enzymes are probably the effective agcnts but ^ve at present 

 have no knowledge eitlier of their source or nature. 



AVe left the pollen grain at a stage when the protoplast, in 

 the form of a lioUow shell, liad enlarged sufficiently to hll the 

 cell-cavity once again. At a slightly later period the genera- 

 tive cell and the vegetative nucleus leave their peripheral posi- 

 tion for one in the centre of the cell cavitv where thev are 

 suspended, together with more or less cytoplasni, by three thick 

 Strands of cytoplasm and öfter several smaller ones as well. 

 The three thick arms of protoplasm extend to the bases of the 

 three interstitial bodies and it is a significant fact that the intine 

 can iii'st be recognised at these spots and that it here attains 

 its greatest thickness (Fig. 28 and 29). It is diflicult to avoid 

 the conclusion that influences of some kind originate in the 

 nucleus and pass along the three arms of cytoplasm to those 

 spots at which ncw cell-wall lamellae are forming but we are 

 at present quite in the dark as to the nature of these in- 

 fluences. 



In still older pollen-gi'ains, measuring from 108 to 112 // in 

 diameter, the intine forms a continuous \ajer over the whole 

 inner face of the wall. It is thick and easily seen at the base 

 of each interstitial body but it is extremely delicate elsewhere 

 and can only be traced as a continuous membrane with some 

 difficulty. 



^The intine gives very clearly the characteristic reactions of 

 a pectic substance but I was not able to demonstrate the pre- 

 sence of cellulose in it with any certainly. 



The interstitial bodies contain one or more yellowish globu- 

 les which usually entangle an air-bubble in them. These globu- 

 les appear to be of an oily nature for they are blackened by osmic 

 acid and they are soluble in absolute alcohol. 



The protoplasm, covered by the intine, now bores its way 

 through the closing disc and enters the interstitial body which 

 it soon entirely fills. I have foUowed this process most com- 

 pletely in the case of Gaiira Lindlieimeri and I will, therefore, 

 refer to this plant in the present description. 



In the quite young pollen grain of Gaura the interstitial 

 bodies are composed of a homogeneous mucilage which in every 

 way resembles that of Oenoihera at a corresponding age. 



