Plants, and Parthenogenesis. 95 



of the transverse section of the mature pollen of Clarkia, as 

 well as, indeed, of Cucwbita and other plants whose pollen is 

 provided with intermediate corpuscles, he is so entirely mistaken 

 with respect to its nature that he attributes the condition to 

 thickening of the inner pollen-coat. 



That these "dot-cells" of Coelebogyne and allied forms, together 

 with the cells containing the fovilla, are, in relation to the 

 extine, cells of the second generation, can be made out only by 

 their developmental history. For, in fact, they mostly retain 

 within the pollen-cell, during the first stage of development, the 

 appearance of secretory cells ; still it is rare that they contain 

 secretions, as happens in Onagraria and Geranium. 



Their function evidently is, either by means of the osmotic 

 action of their contents, or by the transformation into mucus 

 and the consequent swelling-up of their enveloping membrane, 

 to cause the rupture of the extine, which is always very thin 

 above them. Then they in their turn are thrust out or absorbed 

 by the outgrowing intine, to make way for its extrusion. 



Without doubt similar cells are in requisition in all cases (as 

 Fritsche indeed surmised) to effect the perforation of the extine, 

 when the ripe pollen comes into contact with the moisture of the 

 stigma or any other appropriate fluid. 



Besides these cells, which originally held the same genetic 

 position as the " intine," there exist very frequently, within the 

 fluid contents of the pre-formed (parental) sac of the extine, some 

 actually secreting-cells, containing volatile oils and other secreted 

 matters. These vesicles especially contribute to produce the 

 great varieties in the external aspect of pollen, either being so 

 modified in growth as to constitute warts or spines on the sur- 

 face of the extine, which is usually in part simultaneously 

 absorbed, and in part transformed into lignine, or so spread 

 out uniformly over the whole surface of the pollen-grains, 

 except where these are in mutual contact, as to form a sort of 

 epidermal covering over the intine. 



In pollen- grains furnished with folds, the involutions of the 

 extine have no such cellular membrane on their surface. Also 

 in porous pollen-grains, where the pores of the extine are in part 

 due to the presence of " dot- cells," there is not unfrequently an 

 unusual number of them, developed by endogenous growth, 

 which again by their manifold transformations still further 

 multiply the varieties of pollen. 



Mohl, indeed, in his standard work on Pollen (1834), ex- 

 pressed the opinion that " the outer integument of the pollen 

 must be regarded as a structure composed of cells or of their 

 rudiments and of a homogeneous connective tissue." 



Unfortunately this distinguished histologist, misled by the 



