300 Beer, Development of the pollen grain and anther of some Onagraceae. 
tlieir measurements with those described for fresli material. I 
will not however, lay great weight on tlie evidence of the 
microtome sections as, in spite of every precaution, I never suc- 
ceeded in entirely avoiding slirinkage of tlie pollen-protoplast 
even wlien all tlie other cells of tlie anther were un-contracted. 
I will add here a comparison between tbe measurements of the 
pollen from a living antlier with. those of microtome sections: 
1. Fresli material of Oenothera Viennis examined in the 
juice squeezed from the anther: 
Pollen grain =74 y, 
„ cavity = 40 y, 
„ protoplast = 30 y. 
2. Sections of anther of about same age fixed in Flem- 
mings solution: 
Pollen grain B72y.70y.70y.70 u, 
„ cavity = 42 y . 40 y . 40 y . 40 y, 
„ protoplast = 26 y . 28 y 24 y . 22 y. 
Düring the whole time that the protoplast is separated from 
the membrane in this way the latter continues to grow both in 
extent and in thickness. We are at present quite in the dark 
regarding the manner in which this growth takes place but a 
very brief theoretical consideration of the subject will be found 
among the conlusions at tlie end of this paper. 
We must now enquire whence is derived the material 
necessary for this growth. 
There are two sources from which the plastic material of 
the membrane might be derived, viz the protoplast of the 
pollen grain itself or the tapetuni. 
That metabolic processes of no mean Order are taking place 
in the former is evident from a study of the changes which 
can be observed in it during this period. 
Starch appears and disappears in the pollen grain in a 
manner wliich shows that carbohydrates are being used up in 
the cell: the cytoplasm continually grows less and less in amount 
whilst a liquid, apparently the direct consequence of the fore- 
going processes, gradually forms in the protoplasm. 
This liquid first occupies small vacuoles in the cytoplasm. 
tliese continue to increase in size and run together (Fig. 24) 
until we find nothing left of the protoplasm but a hollow shell 
consisting of a plasmoderma (HautSchicht) and a nucleus, sur- 
rounded by a trace of granulär cytoplasm (see Fig. 25 which 
gives a rather later stage). The centre of the shell is occupied 
by one enormous vacuole 1 ). 
There is no reason to doubt that this liquid diffuses out 
from the protoplast into the space which is forming between 
p Strasburger in liis work of 1882 already wrote of Gaura biennis. 
„ln meinen Alkobolpräparaten bildet der nach Anlage der AVand erschöpfte 
Inhalt der Pollenzelle nur noch ein unscheinbares Klümpchen“ cf. his 
Pigs. 47. 48 and 49, Tafel YI. 
