STRUCTURE OF THE ANTHER-WALL. 383 



If we now examine the cross-section with a higher power, we 

 see most externally the epidermis (or exothecium), of flat cells 

 filled with violet cell-sap. These epidermal cells are bulged 

 outwards. At the edges of the partition between the pollen-sacs 

 they are rapidly reduced to a small height. Here the separation 

 from the partition takes place. Stomata are scattered over the 

 whole surface of the anther, and a small air-chamber lies under 

 each of them. To the epidermis succeeds, in the wall of the 

 pollen-sac, a single layer of comparatively high cells, with annular 

 thickenings, the so-called fibrous-layer (or mesothecium). The 

 rings on these cells are arranged perpendicularly to the surface ; 

 they pass over here and there into spiral thickenings, and anasto- 

 mose frequently into a network. Passing towards the dorsal side 

 of the anther the walls of the sac become gradually thicker, the 

 fibrous layer being doubled. The remainder of the body of the 

 anther is likewise constructed of fibrous cells. Only the cells 

 which surround the vascular bundle of the connective, and those 

 (p) which form the partition between the pollen-sacs, are without 

 thickening ridges. 



In order to prepare surface sections of the anther, we again 

 select a flower-bud about two-thirds developed. The surface 

 sections show that over the sacs the epidermal cells are longi- 

 tudinally, the cells of the fibrous layer, on the other hand, are 

 transversely, elongated. Not so on the dorsal surface of the 

 anther, where the fibrous cells appear more isodiametric. Over 

 .the sacs the thickening ridges on the outer side of the fibrous 

 cells are weaker, often Scarcely recognisable. Often in Angio- 

 sperms, as is found also in Taxus, the thickening is entirely 

 wanting on the outer surface of the fibrous cells of the wall of the 

 pollen-sac, so that the thickening ridges show U-shaped figures, 

 open towards the exterior. As in drying the cells of the fibrous 

 layer contract strongly, especially in transverse direction, and are 

 hindered hi radial contraction by the thickening ridges, so there is 

 brought about an unequal contraction of the fibrous layer, re- 

 sulting in the opening of the pollen-sacs. In order to obtain cross- 

 sections of unopened pollen-sacs we must go back to successively 

 younger and younger flower-buds, so long as proves to be neces- 

 sary (Fig. 140, B). 



Development of the Pollen and Pollen-Sac. If we now prepare 

 cross-sections through a flower-bud about J inch high, we shall 

 find the walls of the sacs consisting, besides the epidermis (Fig. 



