484 Prof. H. Karsten on the Formation, 



see the gradual growth of these cells, but, on the contrary, sup- 

 posed, like Mirbel, that the membrane is produced by the solidi- 

 fication of the peripheral layer of the four portions into which 

 the contents of the mother cell are divided. 



In anthers of a rather greater age, the thickening of the 

 membranes of the daughter cells forming the septa commences 

 after the secondary membrane of the mother cell has become 

 considerably thickened and acquired seam-like elevations, like 

 those of the cells of collenchyma, between the pollen-cells which 

 are extending themselves peripherally. 



This thickening is first seen at the periphery, where they and 

 the mother cells are in contact, and in the next place at the 

 centre, where the cell-nucleus was replaced by the mucoid mass. 

 The boundary-line is at first difficult of detection, owing, with- 

 out doubt, to the presence of fluid occupying the intercellular 

 spaces. This is probably the reason why Mirbel considered this 

 portion of the thickened primary pollen-cells as the wall of the 

 pollen mother cell itself (fig. 12). 



When the thickening has further advanced, so as to occupy 

 the entire extent of the cell-membrane, the boundary-line of the 

 contiguous thickened membranes again becomes visible. The 

 origin and the growth of the secondary and succeeding cells of 

 the pollen-cell cannot be traced with certainty. 



The gristly thickened membranes of the pollen mother cell, 

 together with those primary ones of the pollen-cell connected 

 with them (PI. V. fig. 7), become absorbed in the future course 

 of the formation of the pollen. When exposed for some time to 

 the action of water in the state of greatest thickening, and 

 therefore probably at the commencement of resorption, they are 

 burst asunder by the pollen-cells, which then swell forth at those 

 points where the thickening is least, — that is to say, upon the 

 peripheral vertical lines of the pollen-cells. 



The primary cartilaginous thickened cells of the pollen-cells 

 then remain behind, in connexion with the similarly collenchy- 

 matose-looking membranes of the mother cell, in the shape of 

 empty envelopes (fig. 10). 



The still very thin-walled membrane of the pollen-cells, thrust 

 out in this manner (the secondary cells representing in situation 

 the primary membrane), extrudes from the often comparatively 

 small rent, and, by the continuous imbibition of water, attains 

 eventually double its original diameter. The great elasticity of 

 the young and unthickened cell-membrane is displayed in a re- 

 markable manner during this expansion : it is often squeezed 

 through an aperture scarcely one-fourth its own diameter; but, 

 after having effected its escape, it resumes its dimensions and 

 globular form. In immediate contact with its inner wall there 



