720 MICROSCOPIC STRUCTURE OF PHANEROGAMIC PLANTS 



be made up of ordinary cellular parenchyme in which no peculiarity 

 anywhere shows itself; but a gradual differentiation speedily takes 

 place, consisting in the development of a set of very large cells in 

 two vertical rows, which occupy the place of the loctdi or ' pollen- 

 chambers ' that afterwards present themselves ; and these cells give 

 origin to the pollen-grains, whilst the ordinary parenchyme remains 

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

 formed within ' mother-cells,' the eiidoplasm of each breaking up 

 into four segments. These become invested by a double envelope, a 

 firm extine, and a thin intine ; and they are set free, when mature, 

 by the bursting of the pollen-chambers. It 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 struc- 

 ture, though not in form, to the elaters of Marchantia (fig. 506). 

 For they have in their interior a fibrous deposit, which sometimes 

 forms a continuous spiral (like that in fig. 532), 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 it forms an irregular 

 network, as in the violet and saxifrage ; in other cases again, a 

 set of interrupted arches, the fibres being deficient on one side, as in 

 the yellow water-lily, bryoiiy, primrose, &c. ; whilst a very peculiar 

 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 

 intermediate 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 have some share in the opening of the pollen-chambers and 

 in the dispersion of the pollen-grains. 



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 very often 

 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 thimiiiig 

 away at others. Sometimes these markings give to the surface layer 

 so close a resemblance to a stratum of cells (fig. 565, B, C, D) that 

 only a very careful examination can detect the difference. The 

 roughening of the surface by spines or knobby protuberances, as 

 shown at A, is a very common feature ; ajid this seems to enable 

 the pollen-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 



