1198 A TEXTBOOK OF THEORETICAL BOTANY 



occurs, dividing each cell to form a tetrad of pollen grains. The process of 

 pollen-grain development will be described in a subsequent chapter. 



Meanwhile the superficial cells of each lobe have divided tangentially 

 and radially to form a wall-zone, three or four cells thick. This completes 

 the difierentiation of the lobes from the central mass of the connective. 



The superficial layer of cells in the wall of the anther becomes the 

 epidermis but the sub-epidermal cells enlarge to a conspicuous size and 

 usually become cubical or rectangular. The radial walls of these cells are 

 thickened by a reticulum, more or less spiral in pattern, of narrow bands of 

 lignin, leaving the inner and outer tangential walls unthickened. The 

 bands are formed at the expense of starch stored in these cells, and they are 

 arranged in patterns; spiral, U-shaped, ring-shaped, etc., which are 

 characteristic of the family to which the plant belongs. Finally the proto- 

 plasm disappears and the empty cells, forming what is often called the 

 middle layer, constitute, with the epidermis, the wall of the fully matured 

 sporangium (Fig. 1171). 



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Fig. 1 171. — I.iliuiu candiclitm, trans\crse section of an anther showing four lobes with pollen- 

 sacs and connective with vascular bundle. Each pollen-sac is surrounded by a dark 

 zone of tapetum. The line of dehiscence between each pair of pollen-sacs is shown and 

 the large cells of the middle layer in the anther wall. 



