POLLEN OF ANGIOSPERMS. 



761 



of the stigma with pollen-grains must be profuse if all the ovules arc 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 Angiospcrms 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 Slras- 



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

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

 c nectaries ; /wall of carpels ; ir style ; h stigma ; i germinating pollen-grains ; klm ^ pollen-tube which has 

 reached and entered the micropyle of the ovule j n funicle of ovule ; its base ; f outer, g inner integument ; 

 .r nucellus of ovule ; / cavity of the embryo-sac ; ic its basal portion with the antipodal cells ; v synergidK ; s 



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



