146 



MORPHOLOGY OF THE ANGIOSPERMS 



is mature. The spore, after germination, when the first cells of the male 

 gametophyte appear within the wall, becomes the pollen grain. 



A so-called "third layer" of the anther wall — the layer adjacent to the 

 tapetum — has been called the sporangium wall and persists in only 

 some taxa. But this layer is prominent and persists only in thin-walled, 

 highly specialized anthers, not in primitive stamens, where it would be 

 expected to persist if it were a vestige of a true sporangium wall. 



B 



Fig. 60. Scirpus, germination of microspore mother cell, three of the daughter cells 

 degenerate. A, S. lacustris: 1, the four daughter cells within the mother-cell wall; 

 2, one nucleus enlarging, three beginning to degenerate; 3, the surviving cell has 

 divided, the other three reduced to remnants. B, S. uniglumis: 1, surviving daughter 

 cell has divided (surrounded by phragmoplast ) , the other three spores aborted; 2, 

 later stage of B\, vegetative and generative nuclei maturing. {After Piech.) 



