VII.] THE STAMENS. , 77 



the median line of the anther. In some anthers (in 

 the Heaths, Rhododendrons, and nearly all Ericaceae, 

 Solanum, c.), the dehiscence is restricted to short slits 

 or pores at the apex of the anther ; while in others (Bar- 

 berry, Bay Laurel) the portion of the membrane of the 

 anther, on its inner face, overlying the pollen, separates 

 all round each pollen-cavity, excepting at a narrow point 

 near the top, by which it remains hinged, becoming at 

 length sharply recurved, or falling away. 



Pollen * consists in most plants of minute, free, usually 

 oblong ellipsoidal or spherical, double-coated cells, fre- 

 quently marked upon the outer surface with microscopic 

 ridges or tubercles, and capable of opening under favour- 

 able conditions, as, for example, when the pollen falls 

 upon the mature moist stigma, at one or more definite 

 points, to permit the protrusion of the inner coat of the 

 pollen-grain in the form of the so-called pollen-tube a 

 delicate hair-like filament which penetrates, in some 

 plants slowly, in others rapidly, the tissue intervening 

 between the stigma and the cavity of the ovary, where it 

 comes in contact with an ovule. 



The pollen-grains originate by repeated division of the 

 cellular tissue occupying certain positions in the young 

 anther ultimately occupied by the pollen-cavities. At 

 an early stage of development a transverse section of a 

 young anther shows the greater size of the parent-cells 

 of the pollen as compared with the cells of the surround- 

 ing tissue, which forms at length the sides and partitions 

 of the anther. When the division of the parent-cells of 

 the pollen has reached the last stage immediately pre- 

 ceding the formation of the actual pollen-grains, they 

 become more or less free from each other, or at least are 

 easily separable in water. The cell-nucleus is absorbed 

 and replaced by four new nuclei, so disposed in the cavity 

 of the cell that its wall, becoming symmetrically constricted 

 around each nucleus with its surrounding protoplasm, 

 separates the whole into four equal nearly globular por- 

 tions, the pollen-grains ; the outer wall of each grain then 

 acquires its peculiar physical and structural characters, 

 and the pollen is ripe. This is the usual mode of develop- 

 ment, liable to modification in certain groups of plants. 



* Recur to this paragraph after mastering Chapter VIII. 



