264 OROANOGRAPIir. 



becomes more concentrated, and contains less fluid matter and 

 more granules. Some of these granules are said to be no more 

 than about 3^7^ of an incli in diameter, while the largest are 

 about :j(ij^ or -=rj^. Tlicy vary also in form, some being spheri- 

 cal, others oblong, and otliers more or less cylindrical -with 

 somewhat tapering extremities. When water is applied to tlic 

 granular contents they become opaque. When viewed under a 

 high magnifying power, the starch granules at certain periods 

 (especially at the period of dehiscence), exhibit a very active 

 tremulous motion, moving to and fro in various directions and 

 appearing as if repelled by each other. This is simply mole- 

 cular motion, analogous to that of all other very minute par- 

 ticles when suspended in a liquid. The fovilla is Avithout doubt 

 the essential part of the pollen- grain, but the office it performs 

 will be explained hereafter. 



3. Form and Size of ihe Pollen. — Pollen-grains are found of 

 various foi'ms. The most common appear to be the s])herical 

 (^fig. 557), and oval {fig. 554); in other cases they are poly- 

 hedral, as in Chicory and Sonchus palustris, or triangular with 

 the angles rounded and enlarged (trigonal), as in the Evening 

 Primrose and plants generally of the order Onagrariacere (fig. 

 559), or cubic, as in Basella alba, or cylindrical, as in the 

 Tradescantia virgniica, while in Zostera, they are thread-like or 

 of the form of a lengthened tube or cylinder. It sliould be 

 noticed that the form of the pollen is materially influenced ac- 

 cording as it is dry or moist. Thus the pollen-grains of tlie Purple 

 Loosestrife and some species of Passion-flower are oval when 

 dry, but when placed in water they swell and become nearly 

 globular : this arises from endosmotic action taking place 

 between the thickened fovilla and the water, by which 

 some of the latter is absorbed, and the pollen consequently dis- 

 tended. Again, when spherical pollen-grains are exposed to the 

 air for some time they frequently assume a more or less oval 

 form. In size, i)olIen-grains vary from about ^ to J^^ of an 

 inch in diameter ; their size however like their form, is liable 

 to vary according as they are examined in a dry state or in 

 water. 



4 . Dehiscence of the Pollai. — We have already stated, tliat when 

 the ])o]Icn-grains are placed in water they become distended in 

 consequence of endosmotic action taking ])lace between tlieir 

 thickened contents and the surrounding fluid. If this action be 

 continued by allowing the pollen-grains to remain in the liquid, 

 they must necessarily l)urst at some point or other, and allow 

 their contents to cscajte. As the intine is very extensible, 

 while the cxtine is Arm and resisting, it will be found that the 

 former will form little projections through the ])oresor slits of the 

 latter(,^r/. 500), so as to ))roduce little blister-like swellings on its 

 surface. Ultinuitely, however, as absoi*i)tion of fluid by cndos- 



