Beer, Development of tlie pollen grain and anther of some Onagraceae. 307 
In olcler grains this structureless mncilage becomes distinctly 
laminated. These laminae are very closely arranged at the base 
of the interstitial body and form there a closing disc. 1 ) 
Above the closing disc the laminae are much more loosely 
placed and they often become drawn ont and even broken at 
their middle by the growtli of the interstitial body. At the 
apex of the interstitial body the laminae again are very densely 
arranged. 
The intine forms cpiite a thick pad under each interstitial 
body but is very thin over the rest of the pollen grain. It 
contains both cellnlose and a pectic body in its composition. 
Both substances are distributed equally through the thickness of 
the membrane and there is no differentiation of a pectic layer 
from a cellnlose one. 
When the intine and protoplast are about to penetrate the 
interstitial body we first find that a narrow cleft is bored 
through the middle of the closing disc (Fig. 46). Then a small 
fold of intine can be seen to pass into this slit (Fig. 47) and to 
gradually make its way to the centre of the interstitial body 
where the laminae are thin or quite broken through. Here it 
bulges out into a small, thick-walled sac (Fig. 48 and 49). The 
laminae of the interstitial body are gradually eaten away and 
the intine-sac continues to grow until it lies closely against the 
short teeth which alone remain of the interstitial laminae. 
It is interesting to note that the intine must be of a very 
soft and even mucilaginous nature as it often moulds itself to 
all the irregularities on the wall and “flows“ between the teeth 
which project from the interstitial wall. The opening in the 
closing disc gradually enlarges until the disc is reduced to a 
narrow and dense collar or ring (Fig. 50). 
Tlie mariner in which the closing disc is perforated and 
the substance of the interstitial body slowly eaten away suggests 
the presence of a solvent, probably an enzyme, which is secreted 
by the protoplast and which carries out the work of disinte- 
gration. It is difheult otherwise to explain the appearance of 
a clean cut aperture in the closing disc before the intine grows 
out to force itself a way. Moreover, the slow dissolution of the 
interstitial laminae takes place before the intine cornes into 
contact with them so that they cannot be mechanically broken 
down by the growtli of tliat membrane. 
The mature pollen grain of Oenothera longiflora measures 
between 170 and 180 g across; it is quite hlled by the proto¬ 
plast which is densely crowded with starch. The two layers of 
the exine are again in contact with one another. 
The outer layer is, however, only hrmly attached over the 
interstitial bodies; it consists of an outer, liomogeneous lamella 
which is continuous over the whole pollen grain and the inner 
q So closely are tlie laminae arranged in tlie closing disc tliat tlie 
laminated appearance is often lost sight of altogetlier and tlie disc appears 
granulär. 
