520 PAET III. THE CLASSIFICATION OF PLANTS. 



comes disorganised : externally to this is the wall of the pollen- 

 sac consisting of one or more layers of cells with usually reticu- 

 lately thickened walls, followed by the epidermis at the surface. 



The pollen-sacs dehisce usually by a longitudinal slit which, 

 when the anther is quadrilocular, is generally so situated that it 

 at once opens into both the pollen-sacs of each half of the anther, 

 and frequently the tissue separating each pair of pollen-sacs 

 becomes dried up and ruptured whilst the anther is ripening : 

 sometimes the dehiscence of the pollen-sac is transverse (Al- 

 chemilla); sometimes it is valvular (Lauracese, Berberidaceae) ; or 

 by apical pores (Ericacese, Polygalaceae). Though in a quadril ocular 

 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 surface ; 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 ; ex- 

 trorse anthers occur in the Calycanthaceae, Aristolochiaceae, 

 Iridaceae, Juncaginese, Araceae, 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, avimdare, etc.), when the anthers of the outer whorl are 

 introrse, and those of the inner whorl extrorse ; and as in most 

 Lauraceae, 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. 125 and 434). 



The shapes of the pollen -grain are very various (p. 436) : it 

 may be spherical, oval, triangular, etc., or long and cylindrical 

 (confervoid) as in the Naiadacese. In Halophila the shortly cylin- 

 drical pollen-grains adhere so as to form filaments. 



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 present), either rupturing it irregularly, 

 or at determinate points where the exine is thinner and less re- 

 sistent (e.g. Onagracese, Malvaceae), or where there are lid-like 



