'-J'l Beer, Development of the jiolleii graiii aiul imtlicr of some Onagraceae. 



AVlien tetrads at tliis sta,r«:G are treated with 10%KOH the 

 callose wall is dissolvecl aiul tlic pioiopla.sts, each surrounded b}- 

 the undissolved i)ollen membrane, are set free. 



\Vo may coiichide from these reactions that tke young 

 pollen wall is coniposed of a poctic substance. The remains of 

 the primary sporogenous cell wall, which also gives the reactions 

 of a pectic body, can still he distinguished at the periphery of 

 the tetrad. Although the association between the newly deve- 

 loped pollen membrane and the special-mother-cell wall is so 

 dose the demarcation between tho two is always sharp and 

 there is never a gradation of oiiu into the otlier. AVhere the 

 callose wall abuts upon the pectic membrane it is denser and 

 refracts the liglit more strongly than the rest of the wall pro- 

 bably forming there a „Grenzhäutchen" in Strasburgers 

 sense. 



The facts sliow that although the pollen w^all is at first 

 deposited in close contact witli, and probably in actual attach- 

 ment to, the special-mother-cell wall it is not derived from a 

 transformation of the innermost lamella of tliis callose wall but 

 is directly secreted as a pectic layer by the cell-protoplast. It is 

 equally certain that the plasmoderma ^) is not bodily transformed 

 into the pollen wall but that this is de]30sited upon the surface 

 of the plasmoderma, as Strasburger has shown in other casese. 



As soon as the pollen membrane becomes slightly thicker 

 it separates readily from the callose layers. and is then clearly 

 recognised as an independent structure. The young pollen grain 

 is a blimtly triangulär, basin-shaped structure with the concavity 

 of the „basin" directed towards the centre of the tetrad. It 

 measures about 19 to 20 /i across its broadest surface in 0. biennis. 

 At the apex of the tliree angles of the pollen grain the wall is 

 extremely thin. The protoplasm fills the cell-cavity and con- 

 tains a considerable quantity of starcli (Figs. 15 and 16). 



In pollen grains which are a little older [mearuring about 

 22 to 24 // across in 0. hiennis'^)] the wall has thickened con- 

 siderably and a mucilaginous material has been developed at 

 the three ungles of the cell at those spots which previously were 

 thin (Fig. 17). This mucilage gives the reactions of a pectic 

 substance and appears to he derived from the growtli and phy- 

 sical alteration of the pollen Avall at these points. The little 

 plugs or discs of mucilage continue to enlarge and soon bulge 

 so far within the cell that they overlap the unswoln poUen 

 wall on each side. 



1) I use this term as the equivalent of the german „Hautschicht". The 

 word -vvas suggested for this purpose by Strasburger and first iised by 

 Stevens in his paper upon „Gametogenesis and Fertilization in Albugo", 

 (Bot. Gazette XXXII. 1901. p. 92.) 



2) In stating the size of the pollen grain I have alwaj^s taken the 

 measui-ement across the broad face of the grain from the'tip of an inter- 

 stitial body to the outer surface of the wall imniediatly ojjposite. 



