398 



STRUCTURE OF PHANEROGAMIC PLANTS. 



more, however, when the pollen is dry, than when it is moist ; 

 for the effect of the imbibition of fluid, which usually takes place 

 when the pollen is placed in contact with it, is to soften down 

 angularities, and to bring the cell nearer to the typical sphere. 

 The pollen-cell (save in a few submerged plants) has a thick 

 outer coat, surrounding a thin interior wall; and this often 

 exhibits very curious markings, which seem due to an increased 

 thickening at some points, and a thinning away at others. Some- 

 times these markings give to the surface-layer so close a resem- 

 blance to a stratum of cells (Fig. 189, B, c, D), that only a very 

 careful examination can detect the difference. The roughening 

 of the surface by spines or knobby protuberances, as shown at A, 

 is a very common feature ; and this seems to answer the purpose 

 of enabling the pollen-grains more readily to hold to the surface 

 whereon they may be cast. Besides these and other inequalities 

 of the surface, most pollen-grains have what appear to be pores 

 or slits in the outer coat, varying in number in different species, 

 through which the inner coat protrudes itself, when the bulk of 

 its contents has been increased by imbibition ; it seems probable, 

 however, that the outer coat is not absolutely deficient at these 

 points, but is only thinned away. Sometimes the pores are 

 covered by little disk-like pieces, or lids, which fall off when the 

 pollen-tube is protruded. This action takes place naturally, when 

 the pollen-grains fall upon the surface of the "stigma," which is 

 moistened with a viscid secretion ; and the pollen-tubes, at first 



mere protrusions of the inner 

 coat of their cell, insinuat- 

 ing themselves between the 

 loosely packed cells of the 

 stigma, grow downwards 

 through the "style," some- 

 times even to the length of 

 several inches, until they 

 reach the ovarium. The first 

 change, namely, the pro- 

 trusion of the inner mem- 

 brane through the pores of 

 the exterior, may be made 

 to take place artificially, by 

 moistening the pollen with 

 water, thin syrup, or dilute 

 acids (different kinds of pol- 

 len-grains requiring a differ- 

 ent mode of treatment) ; but 

 the subsequent extension by 

 growth will only take place under the natural conditions. 



254. The darker kinds of pollen may be best mounted for the 

 Microscope in Canada balsam ; but this renders the more trans- 

 parent kinds too faintly distinguishable ; and it is better to 



FIG. 189. 



Pollen grains of, A, Alihcea rosea; B, Cobcea 

 scandens; c, Passifl&ra. ccerulea ; D, Ipomcea pur- 

 purea. 



