Beer, Development of tlie poUen grain and antlier of some Onagraeeae. 309 



gularities in the divisions similar to tliose described by Juel in 

 HemerocalVis fulva. 



■4-. The first poUen luenibrane is fornled by the direct 

 activity of the protoplast and is deposited as a deHcate layer of 

 pectic material upon the inner face of the special-niother-cell 

 wall. Althongh it originates in the most intimate contact with 

 the callose wall it is chemically distinct from this from the very first. 



5. The interstitial bodies originate as specialised areas on 

 the first pollen wall. These spots are at first characterised by 

 their gi-eater thinness; later a homogeneous mucilage is develo- 

 ped at these places. In older jjollen grains a portion of this 

 mucilage is deposited as a dense closing disc whilst, in Ocnothera, 

 the rest of the interstitial body is filled with a thin fluid. In 

 Oaura more or less solid laminae are deposited throughout the 

 interstitial body. 



6. A secondary thickening layer is laid down by the proto- 

 plast within the first pollen niembrane. This layer gives most of 

 the pectic reactions but also a very distinct violet colour with a 

 strong Solution of lodine in potassium iodide. It gives none of 

 the usual cellulose reactions. 



7. Both the first pollen wall and the secondary thickening 

 layer are firmly attached to the protoplast wlien they are first 

 developed. 



In pollen grains which have reached 40 ^ in size the proto- 

 plast is no longer fixed to the wall at any place although it still 

 completely fiUs the cell cavity. 



The pollen grain continues to grow and its walls increase 

 both in thickness and in extent. "Whilst the pollen grain doubles 

 its diameter the cell-cavity increases in size from about 26 // 

 to about 46 jx. The protoplast, however, grows far less rapidly 

 dm-ing this time and its diameter only enlarges from 26 ii to 

 about 34 //. 



In consequence of this inequality in growth the protoplast 

 becomes separated from the pollen wall by a space which is 

 filled with liquid. 



The conditions seem to be quite similar to those which 

 Fitting and others have described in the megaspores of Isoetes 

 and SelagincUa. 



In Oenofhera also, as in the megaspores, the growth of the 

 layers of the wall is not equally rapid and the first pollen wall 

 becomes separated from the secondary thickening layer and is 

 thrown into folds upon its surface. 



These observations show tliat during the period of most 

 active growth of the membrane (both in surface and in thick- 

 ness) the protoplast is completely separated from it and we 

 must conclude either that the growth of a coll-waU is a purely 

 physical process or that the living protoplast can exert its in- 

 fluence across a space filled with liquid. 



I may add here that, although the growth of a membrane 

 whilst this is separated from the protoplast by an actual space 



Beihofte Bot. Centralbl. Bd. XIX. Abt. I. Heft -2. 21 



