ANGIOSPERMS. 



most layer of the concentric system of wall-layers, as is shown clearly by its 

 separation from it after contraction in alcohol; this is the true wall of the 

 pollen-grain, and it now becomes rapidly thicker and differentiated into an outer 

 cuticularised layer, Jhe exine, and an 

 inner one of pure cellulose, the 

 intine\ the former becomes covered 

 on the outer surface with spikes (Fig. 

 294 ph\ warts, ridges, combs, &c., 

 while the latter often forms consi- 

 derable thickenings, which project 

 inwards at certain spots (Fig. 294 v), 

 and take part at a later period in the 

 formation of the pollen-tube. During 

 these processes the layers of cellulose 

 surrounding the tetrads slowly dis- 

 solve, their substance is converted 

 into mucilage and their form finally 

 disappears ; their disorganisation may 

 commence on the inner side of the 

 wall of the mother-cell (Fig. 289 

 VII, x) or on the outer side (Fig. 

 294 sg). By the dissolution of the 

 chambers in which the young pollen- 

 grains were till now enclosed, they 

 are set at liberty and separate from 

 one another and float in the gra- 

 nular fluid which fills the cavity of 

 the loculament, and there attain to 

 their ultimate development and size ; FIG. 293 . cucurbtta Pejo. A a poiien-gram putting out its tube SJ > 



,. ,1 n .j j which is penetrating into a papilla of the stigma np. The inline is much 



in thlS prOCeSS the flUld IS USed Up, thickened at certain spots Bt', the exine forms a round lid rf upon each 



j v . 11 thickening-mass ; when the grain prepares to germinate, the thick layers 



and tile ripe pOlien-gramb are at O f the inline swell, and lift off the piece of exine which forms the lid ; 



length a powdery mass filling the SSS^S*" 

 anther-chamber. 



Similar processes take place in the ripe pollen-grains or microspores of the 

 Angiosperms to those with which we are acquainted in the microspores of the Gym- 

 nosperms, as Strasburger has recently discovered 1 . The pollen-grain either imme- 

 diately after its formation, or at some later time but always before pollination, becomes 

 divided into two cells, a larger cell and a smaller 'vegetative' or prothallium-cell 



1 Strasburger, Ueber Befruchtung und Zelltheilung, Jena, 1878. Elfving, Studien ii. d. Pollen- 

 korner d. Angiospermen (Jen. Zeitschr. f. Naturw. Bd. XIII, N.F. Bd. VI). On the external sculptur- 

 ing etc. see Schacht in Jahrb. f. wiss. Bot. II. 149, and Lurssen in the same publication, VII. p. 34. 

 [Strasburger's more recent views regarding the homologies of the pollen-grain and the nature of the 

 processes which go on within it are referred to in a note on page 310. In the case of Angiosperms 

 the small cell, the so-called prothallium-cell, is, he maintains, the progamous cell, its nucleus 

 combining with the nucleus of the oosphere, and it will therefore be the homologue of the large cell 

 in Gymnosperms.] 



