166 



MORPHOLOGY OF THE ANGIOSPERMS 



escape. In fusion chambers, the pollen grains probably do not inter- 

 mingle, except as they escape. 



Dehiscence is not always due to changes in the fibrous layer, for 

 this layer is not always present, especially in poricidal anthers. Details 

 of dehiscence in anthers of this type vary considerably, but the pores 

 usually form by disintegration of the anther wall in a small area, with 

 shrinkage of the surrounding tissues. Rarely, a slit occurs in tlie epi- 



Fig. 67. Degeneria vitiensis showing development of microsporangia, spores, and 

 pollen grains. A, B, groups of archesporial cells in cross section of the lamina; C, D, 

 organization of the tapetum; E, sporangia with microspores and the beginning of 

 endothecial caps, tapetum degenerated; F, pair of sporangia united by breakdown 

 of intermediate tissue, two-celled pollen grains, and endothecium; G, microspore 

 with germination furrow, dividing; H, pollen grain at shedding stage, generative 

 cell dark, tube nucleus Hghter; /, young tetrad in mother-cell wall, showing initia- 

 tion of furrow on distal face of grains; /, pollen grains of a tetrad, just separated, 

 showing furrows; K, germinating pollen grain with tube projecting from the fur- 

 row. (After Swamtj.) 



