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. 4 
The gristly thickened membranes of the pollen mother cell, — 
together with those primary ones of the pollen-cell connected _ 
with them (Pl. 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 — 
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