ANGIOSPERMS. 



367 



grain depends chiefly on the number and arrangement of the points of exit, and 

 on the condition and behaviour of the exine at these points ; whether it is simply 

 thinner there while the intine protrudes as a wart (Fig. 296), or whether roundish 

 pieces like lids are detached from it, as in the Cucurbitaceae (Fig. 293), or Passiflora^ 

 or whether it splits up into ribands by spiral fissures, as in Thunbergia (Fig. 295), and 

 so on. The intine is usually thicker at the points of exit, and often forms semi- 

 circular protuberances which supply the first material for the formation of the tube 

 (Fig. 296 /), or the exine only forms thinner longitudinal striae which are furrows in 

 the dry pollen-grain, as in Gladiolus, Yucca, Helleborus, and others. But in many 

 plants the intine is uniformly and continuously thickened, as in Canna, Strelitzia 



FIG, 296. Pollen-grain of Epilobnim a>i- 

 gustifolhim in optical transverse section ; a a a 

 points of protrusion of the intine i which is 

 there thickened, while the exine e is thinner at 

 the same spots. Magn. 590 times. 



JKs 



FIG. 297. Pollen-grain of Althaea rosea. A portion of the exine seen from without. K the half of a very 

 thin aequatorial section of the grain, st large spikes, fCs small spikes of the exine, o holes in the exine, e the exine, 

 i the intine, / the protoplasm of the grain retracted from the intine. Magn. 800 times. 



Musa, Psrsea, and then according to Schacht no points are prepared beforehand for 

 the exit of the tube. The number of these peculiarly organised points of exit is a 

 fixed one in each species, and often in whole genera and families ; one in the 

 majority of Monocotyledons and in a few Dicotyledons ; two in Ficus, Justicia, and 

 some others ; three in the Onagrarieae, Proteaceae, Cupuliferae, Geraniaceae, Com- 

 positae, and Boragineae; from four to six in Impato'ens, Astrapaea, Alnus, and 

 Carpinus', many in the Convolvulaceae, Malvaceae, Alsineae, etc. (see Schacht loc. cit.). 

 The exine is not often smooth, and has usually the sculpturing mentioned above on 

 its outer surface. If it is more than usually thick, it often shows layers of different 

 structure and consistence, and differentiations sometimes appear in it passing through 



