256 ORGANOGRAPHY. 



Some of these granules are no more than about go^oo °^ ^^ ^"^^h 

 in diameter, while the largest are about ^^ or -^-^^q. They 

 vary also in form, some being spherical, others oblong, and 

 others more or less cylindrical with somewhat tapering extremi- 

 ties. When water is applied to tlie granular contents they be- 

 come opaque. When viewed under a high magnifying power, 

 the starch granules at certain periods (especially at the period of 

 the dehiscence of the pollen), exhibit a very active tremulous 

 motion, moving to and fro in various directions and appearing as 

 if repelled by each other. This is simply molecular motion, 

 analogous to that of all other very minute particles when 

 suspended in a liquid. The fovilla is without doubt the essen- 

 tial part of the pollen-cell, but the office it performs will be ex- 

 plained hereafter. 



3. For)n and Size of the Pollen. — Pollen-cells are found of 

 vai'ious forms. The most common forms appear to be the spherical 

 (figs. 28 and 553), and oval (j?^. 554); in other cases the pollen- 

 cells are polyhedral, 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 Ona- 

 gracese (fig. 559), or cubical, as in Basella alha, or cylindrical, 

 as in Trach scantia virginica, while in Zostera they are thread- 

 like or of the form of a lengthened tube or cylinder, and other 

 forms also occur. It should be noticed that the form of the 

 pollen is materially influenced according as it is dry or moist. 

 Thus the pollen-cells of the Purple Loose-strife and some species 

 of Passion-flower are oval when dry, but when placed in water 

 they swell and become nearly globular : this arises from endos- 

 motic action taking place between the thickened fovilla and the 

 water, by which some of the latter is absorbed, and the pollen 

 consequently distended. Again, when spherical pollen-cells are 

 exposed to the air for some time they frequently assume a more 

 or less oval form. In size, pollen-cells vary from about gso ^ 

 Y^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 Pollen. — We have already stated that 

 when the pollen-cells are placed in water they become distended 

 in consequence of endosmotic action taking place between their 

 thickened contents and the surrounding fluid. If this action be 

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

 they must necessarily biu'st at some point or other, and allow 

 their contents to escape. As the intine is very extensible, while 

 the extine is firm and resisting, it will be found that the former 

 will form little projections through the pores or slits of the 

 latter, so as to produce little blister-like swellings on its surface 

 {fig. 560). Ultimately, however, as absorption of fluid by en- 

 dosmose still goes on, the intine will itself burst and discharge 



