Beer, Development of the pollen grain and antlier of snme Onagraceae. oOt 



In olcler grains this structureless mucilage becomes distinctly 

 laminated. These laminae are very closely arranged at the base 

 of the interstitial body and form there a closing disc.i) 



Above the closing disc the himinae are mach more loosely 

 placed and they often become drawn out and even broken at 

 their middle by the growth of the interstitial body. At the 

 apex of the interstitial body the laminae again are very densely 

 arranged. 



The intine forms quite a thick päd iinder each interstitial 

 body but is very thin over the rest of the pollen grain. It 

 contains both cellulose 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 cellulose one. 



Wlien the intine and protoplast are about to penetrate the 

 interstitial body we iirst 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 niake 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-waUed sac (Fig. 48 and 49). The 

 laminae of the interstitial body are gradually eaten away and 

 the intine-sac contiiiues to grow until it lies closely against the 

 short teeth which alone remain of the interstitial laminae. 



It is interestine; to note that the intine must be of a very 

 soft and even mucilaginous nature as it often moulds itself to 

 all the iiTeo-ularities on the wall and "flows" between the teeth 

 which iDroject from the interstitial wall. The opennig m the 

 closing disc gradually enlarges until the disc is reduced to a 

 narrow and dense collar or ring (Fig. 50). 



The manner 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 difficult 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 comes into 

 contact with them so that they cannot be mechanically broken 

 down by the growth of that membrane. 



The mature pollen grain of Oenothera hngtflora measures 

 betweon 170 and 180 // across: it is quite filled 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 firmly attached over the 

 interstitial bodies; it consists of an out er, homogeneous lamella 

 which is continuous over the whole pollen grain and the inner 



') So closely are tlie laminae arranged in the closing disc that the 

 laminated appearance is often lost sight of altogether and the disc appears 



granulär. 



