384 THE MICROSCOPE AND ITS REVELATIONS. 



large cells in two vertical rows, which occupy the place of the loculi or 

 ' pollen-chambers ' that afterwards present themselves; and these cells 

 give origin to the pollen-grains, whilst the ordinary parenchyma remains 

 to form the walls of the pollen-chambers. The pollen-grains are formed 

 within ' mother-cells,' the endoplasrn of each breaking up into four seg- 

 ments. These become invested by a double envelope, a firm extine, and 

 .a thin inline ; and they are set free, when mature, by the bursting of the 

 pollen-chambers. Ifc is not a little curious that the layer of cells which 

 lines the pollen-chambers should exhibit, in a considerable proportion of 

 plants, a strong resemblance in structure, though not in form, to the 

 elaters of Marchantia (Fig. 218). For they have in their interior a 

 fibrous deposit; which sometimes forms a continuous spiral (like that in 

 Fig. 244), as in Narcissus and Hyoscyamus; but it is often broken-up, as 

 it were, into rings, as in the Iris and Hyacinth; in many instances forms 

 an irregular network, as in the Violet and Saxifrage; in other cases, again, 

 forms a set of interrupted arches, the fibres being deficient on one side, 

 -as in the Yellow Water-lily, Bryony, Primrose, etc.; whilst a very pecu- 

 liar stellate aspect is often given to these cells, by the convergence of the 

 interrupted fibres towards one point of the cell-wall, as in the Cactus, 

 Geranium, Madder, and many other well-known plants. Various inter- 

 mediate modifications exist; and the particular form presented often 

 varies in different parts of the wall of one and the same anther. It seems 

 probable that, as in Hepaticse, the elasticity of these spiral cells may 

 Jiave some share in the opening of the pollen-chambers and in the dis- 

 persion of the pollen-grains. 



387. The form of the pollen-grains seems to depend in part upon the 

 mode of division of the cavity of the parent-cell into quarters; generally 

 speaking it approaches the spheroidal, but it is sometimes elliptical, and 

 sometimes tetrahedral. It varies 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 extine or outer coat of the pollen-grain often exhibits very 

 curious markings, which seem due to an increased thickening at some 

 points, and a thinning-away at others. Sometimes these markings give 

 to the surface-layer so close a resemblance to a stratum of cells (Fig. 277, 

 B, c, D), that only a very careful examination can detect the difference. 

 Tke roughening of the surface by spines or knobby protuberances, as 

 .shewn at A, is a very common feature; and this seems to enable the pol- 

 len-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 their extine (varying in number 

 in different species), through which the intine protrudes itself as a tube, 

 when the bulk of its contents has been increased by imbibition; it seems 

 probable, however, that the extine 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 pro- 

 truded. This action takes place naturally when the pollen-grains fall 

 upon the surface of the stigma, which is moistened with a viscid secre- 

 tion; and the pollen-tubes, at first mere protrusions of the inner coat of 

 their cell, insinuating themselves between the loosely-packed cells of the 

 stigma, grow downwards through the style, sometimes even to the length 

 of several inches, until they reach the ovarium. The first change 

 namely, the protrusion of the inner membrane through the pores of the 



