296 THE FLOWER. 



grow as one compound grain, without undergoing complete di- 

 vision. The grains of the pollen of the Evening Primrose Fam- 

 ily (Fig. 419) thus consist of the rudiments of four, which remain 

 in strict combination ; one of them enlarging to form the main 

 body of the grain, while the three others appear as bosses on its 

 angles. Rarely the four cohering grains are placed in the same 

 plane. They usually stand in the same relation to each other as 

 the four angles of a cube. In the Mimosa Family, the division 

 goes farther, and gives rise to eight or sixteen lightly coherent 

 grains in each mass. 



535. The pollen-grains have two coats ; the exterior of which, 

 called the extine, is quite firm and often wax-like, granular, or 

 fleshy ; to it the bands, points, or other markings belong. It is 

 thought by Schleiden and others to be a secretion from the inner 

 layer, which, on this view, is considered as the proper membrane 

 of the cell. This inner coat, named the inline, is very thin, trans- 

 parent, and highly extensible. It absorbs water rapidly, and when 

 exposed to its action the grain swells and soon bursts, discharging 

 its contents. These contents are a fluid, which appears slightly 

 turbid under the higher powers of ordinary microscopes, but, when 

 submitted to a magnifying power of three hundred diameters, it is 

 found to contain a multitude of minute particles (foviHa) of spher- 

 ical or oblong form, the larger of which are from the four-thou- 

 sandth to the five-thousandth of an inch in length, and the smaller 

 only one fourth or one sixth of this size. The smaller exhibit the 

 constant molecular motion of all such minute particles when sus- 

 pended in a liquid and viewed under a sufficient magnifying power. 

 The larger are now thought by some to be substantially of the na- 

 ture of starch-grains. A third, intermediate membrane has been 

 detected in certain cases. The pollen of some plants that of Zos- 

 tera very distinctly has only a single (the internal) membrane. 



536. When wetted, the grains of pollen promptly absorb water 

 by endosrnosis (37), and are distended, changing their shape some- 

 what, and obliterating the longitudinal folds, one or more in num- 

 ber, which many grains exhibit in the dry state. Soon the more 

 extensible and elastic inner coat inclines to force its way through 

 the weaker parts of the exterior, especially at one or more thin 

 points or pores. The absorption continuing, the distention soon 

 overcomes the resistance of the inner coat, which bursts, with the 

 eruption of the contents in a jet. When the pollen falls upon the 



