ANGIOSPERMS. 36^, 



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

 separation from it afier 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 



C^ 



inner one of pure cellulose, the 

 I'ntinc; 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 teirads slowly dis- 

 solve, their substance is convevteii 

 into mucilage and their form finally 

 disappears; their disorganisation may 

 commence on the inner side of the 

 wall of the mother-cell (Fig. 289 

 F//, .v) or on ihe 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 ; 

 in this process the fluid is used up, 

 and the ripe pollen-grains are at 

 length a powdery mass filling the 

 anther-chamber. 



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

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

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

 diately after its formation, or at some later time but always be'bre pollination, becomes 

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



FIG. 293 



vhich i 



poUen-ftrain putting 



be .1/ 

 penetrating into a papilla of the stigma «/>. The intine is much 

 thickened at certain spots B i ; the exine forms a rnund lid d upon each 

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

 of the intine swell, and lift off the piece of exine which forms the lid ; 

 pollen tubes are formed from one or two of these thickening-masses. 

 Magn. 550 times. 



' Strasburger, Ueber 15efnichtung und ZcUtheilung, Jena, 1878. — Elfving, .Stiulien ii. d. Pollen- 

 korner d. Angiosperrnen (Jen. Zeitschr. f. Naturw. Bd. XIII, N.F. Bd. VI).— On the e.\tcrnal 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 Angii)S[Kims 

 the small cell, the so-called prothallium-cell, is, he maintains, the prog.imous cell, its nucleus 

 combining with the nucleus uf the oosphere, and it will thLiefoic be the hoinologue of the large cell 

 in Gymnosperms.] 



