POLLEN OF ANGIOSPERMS. 'j6l 



of the stigma with pollen-grains must be profuse if all the ovules are to be fertilised, 

 and since not every pollen-tube accomplishes its end, even a greater number of pollen- 

 grains are needed. 



Also in the pollen-grains of the Angiosperms there is still found a last remnant 

 (one might almost say a feeble and indistinct memento) of the cell-formation which 

 occurs in the microspores of the Cryptogams and Gymnosperms. It is to Stras- 



FIG. 442.— Diagram of a very simple flower in longitudinal section, a transverse section of an anther 

 before its dehiscence ; b an anther dehiscing longitudinally, with pollen ; c filament ; rf base of floral leaves ; 

 * nectaries-iywall of carpels ; ^ style ; A stigma ; i germinating poUen-grains ; .fe ^ »« a pollen-tube which has 

 reached and entered the micropyle of the ovule ; « funicle of ovule ; a its base \ pouter, g inner integument ; 

 s nucellus of ovule J t cavity of the embryo-sac ; « its basal portion with the antipodal cells ; v synergida: ; z 

 oosphere. 



burger's investigations, which have been of such immense service to the theory of 

 fertilisation, that we owe the knowledge of the remarkable fact that the ripe 

 pollen-grains of the Angiosperms regularly contain two cell-nuclei, and that 

 sometimes indeed, a division, although transitory, of the contents is indicated. 



The pollen-grains of the Angiosperms differ much in form and size. The 

 usually thick cuticularised external membrane, the so-called Extine, very often shows 

 beforehand the spots from which the pollen-tube or tubes are to emerge later. As 

 in. the germination of spores generally, it is the second membrane consisting 



