306 Beer, Development of tlie pollen grain and antlier of some Onagraceae. 
walls become very thin and at the time wlien tlie tapetnm 
disintegrates tliey become so attenuated tliat at some spots thev 
are apparently interrupted altogether. It is obvious theref'ore 
that tbe tapetal walls offer no great liinderance to tlie passage 
of tlie cell-contents. It is more difficult, however, to understand 
liow tbe tapetal snbstance passes tbrongb tlie tbick, cuticnlarised 
pollen-wall to reacb tlie protoplast. It must evidently do so in 
a state of solution but bow tbe complex material of tlie tapetal 
cells is brougbt into solution can at present only lie guessed at. 
Enzymes are probably tlie effective agents but we at present 
bave no knowledge eitber of tlieir source or nature. 
We left tbe pollen grain at a stage wlien tbe protoplast, in 
tbe form of a liollow sliell, bad enlarged sufficiently to Eli tbe 
cell-cavity once again. At a sliglitly later period tlie genera¬ 
tive cell and tbe vegetative nucleus leave tlieir peripberal Posi¬ 
tion for one in tlie centre of tbe cell cavity wliere tliev are 
u tj 
suspended, togetlier witb more or less cytoplasm, by tbree tbick 
strands of cytoplasm and öfter several smaller ones as well. 
Tbe tbree tbick arms of protoplasm extend to tbe bases of tbe 
tbree interstitial bodies and it is a significant fact tliat tbe intine 
can first be recognised at tbese spots and tliat it bere attains 
its greatest tliickness (Fig. 28 and 29). It is difficult to avoid 
tbe conclusion tliat influences of some kind originate in tbe 
nucleus and pass along tlie tbree arms of cytoplasm to tbose 
.spots at wliicli new cell-wall lamellae are forming but we are 
at present Suite in tbe dark as to tbe nature of tbese in¬ 
fluences. 
In still older pollen-grains, measuring froni 108 to 112 u in 
diameter, tbe intine forms a continuous layer over tbe wliole 
inner face of tlie wall. It is tbick and easilv seen at tbe base 
of eacli interstitial body but it is extremely delicate elsewliere 
and can only be traced as a continuous membrane witb some 
difficulty. 
Tbe intine gives very clearly tbe cbaracteristic reactions of 
a pectic substance but I was not able to demonstrate tbe pre- 
sence of cellulose in it witb any certainly. 
Tbe interstitial bodies contain one or more yellowisli globu- 
les wliicli usually en'tangle an air-bubble in tliem. Tbese globu- 
les appear to be of an oily nature for tliey are blackened by osmic 
acid and tbey are soluble in absolute alcoliol. 
Tbe protoplasm, covered by tbe intine, now bores its way 
tbrougli tlie closing clisc and enters tbe interstitial body wbicli 
it soon entirely fills. I bave followed tliis process most com- 
pletely in tlie case of Gaura Lindheimeri and I will, tlierefore. 
refer to this plant in tlie present description. 
In tlie quite young pollen grain of Gaura tbe interstitial 
bodies are composed of a liomogeneous mucilage wliicb in every 
wa}^ resembles tliat of Oenoihera at a corresponding age. 
