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, the exine, and an 

 inner one of pure cellulose, the 

 inline ; the former becomes covered 

 on the outer surface with spikes (Fig. 

 294 />//), 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 . cucurbit* Pep a . A a poiien-grain putting out its tube *p 



. , . i fl *J J which is penetrating into a papilla of the stigma np. The inline is much 



m thlS prOCeSS the flUld IS USed Up, thickened at certain spots i; the exine forms a round lid rf upon each 



j 1 -11 thickening-mass; when the grain prepares to germinate, the thick layers 



and the ripe pOllen-grainS are at O f the intine swell, and lift off the piece of exine which forms the lid ; 



i /MV .1 pollen tubes are formed from one or two of these thickening-masses. 



length a powdery mass filling the Magn. sso times, 

 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. Natunv. Ed. XIII, N.F. Bd. VI). On the external sculptur- 

 ing etc. see Schacht in Jahrb. f. wiss. Bot. II. 149, and Liirssen 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.] 



