ANGIOSPERM^: 



sion (Fig. 318) in the cell may be followed by a division-wall (many 

 Monocotyledons), or more commonly the division-walls are not formed 

 until after the second mitosis, and the resulting spores are of the 

 tetrahedral type. In aquatic forms the spore-membrane may remain 

 thin and uncuticularized ; but usually there is an outer thickened 

 perinium like that found in the spores of the Archegoniates. The 

 pollen-spore begins its germination within the pollen-sac. A small 

 cell, the antheridial cell, is cut off (Fig. 318, E), and the nucleus of 

 this subsequently divides into the two generative nuclei. In excep- 

 tional cases e.g. Sparganium simplex (Fig. 318, G) a small sterile 

 cell is cut off from the spore before the antheridial cell is formed. 



FIG. 318. A-C, Allium Canadense. First nuclear, division of the pollen mother- 

 cell (X 400). D-F,Naiasflexilis. J), young pollen-tetrad (X 200). U, pollen-spore 

 with antheridial cell (X400). F, older pollen-spore, with two generative nuclei, 5. 

 G, Sparganium simplex, pollen-spore with sterile prothalliar cell, pr ( X 400) . 

 H, Lathyrus odoratus, germinating 'pollen; x, generative nucleus. 



The anther is usually composed of four pollen-sacs, and the walls 

 of the nearly ripe sporangium consist of three layers of cells, of 

 which the inner one at maturity becomes more or less completely 

 disorganized. The middle one of these thin layers develops upon its 

 walls thickened bands (" fibrils "), which are hygroscopic, and by 

 their contraction effect the dehiscence of the pollen-sacs. They 

 closely resemble the similar spiral thickenings found in the pollen- 

 sacs of many Gymnosperms and in the sporangium of Equisetum. 

 Within the three layers of cells forming the Avail of the sporangium 

 is a layer of tapetal cells, which is broken down before the division 

 of the spores begins. 



2A 



