464 PART IV. CLASSIFICATION. 



comes dried up and ruptured whilst the anther is ripening : some- 

 times the dehiscence of the pollen-sac is transverse (Alchemilla) ; 

 sometimes it is valvular (Berberidacese) ; or by apical pores 

 (Ericaceae, Polygalacese). Though in a quadrilocular anther the 

 pollen-sacs typically belong, two to the inner (ventral), two to the 

 outer (dorsal), surface of the anther, it frequently happens that in 

 the course of their development they become somewhat displaced, 

 so as to appear all to belong to either the inner or the outer sur- 

 face ; hence, when dehiscence takes place, the pollen is shed, in the 

 former case, towards the centre of the flower, when the anthers are 

 said to be introrse ; and, in the latter case, towards the periphery 

 of the flower, when the anthers are said to be extrorse. These 

 terms are similarly applicable in the case of bilocular anthers. 

 Introrse anthers are the more common ; extrorse anthers occur in 

 the Aristolochiacese, Iridacese, Juncaginese, Aracese, and in various 

 genera of other orders. In rare cases some of the anthers of the 

 flower are introrse, and others extrorse, as in some species of 

 Polygonum (P. Bistorta, tataricum, aviculare, etc.), where the 

 anthers of the outer whorl are introrse, and those of the inner 

 whorl extrorse ; and as in most Lauracese, where the anthers of the 

 innermost staminal whorl are extrorse, whilst those of the outer 

 whorls are introrse. 



The Microspores or Pollen- grains. The essential features in the 

 structure and development of the microspores have been already 

 fully described (see pp. 85 and 396). 



The shapes of the pollen-grain are very various : it may be 

 spherical, oval, triangular, etc., or 

 long and cylindrical (confervoid) 

 as in the Naiadacese. 



On germination the pollen- 

 grain gives rise to one or more 

 pollen-tubes, which consist of 

 outgrowths of the intine : these 

 penetrate the exine (when pre- 

 FIG. 2so. Germinating pollen-grain of sent), either rupturing it irregu- 



Epilobium (highly mag.) bearing a , , j , . . , 



pollen-tubes;* exine ;,' intine ; a b c the larl J> Or at determinate points 



three spots where the exine is thicker in where the exine is thinner and 



Malvaceae), or where there are 

 lid-like areas which are easily removed (e.g. Cucurbitacese, 

 Fig. 235). These points are definite in number (1, 2, 3, 4, or 

 more), sometimes very numerous (Malvaceae). 



