ANGIOSPERMS, 
555 
form and sculpture of the outer coat of pollen-grains depends chiefly on the number 
of the spots at which the perforation may take place, on the mode in which these are 
arranged, and on the circumstance whether the extine is at these spots merely thinner 
and the intine projects in the form of a wart (Fig. 380), or whether roundish pieces 
of the extine become detached in the form of a lid, as in Cucurbitacese and Passiflora 
(Fig. 35, p. 33), or whether it splits into bands by spiral fissures, as in Thunbergia 
(Fig. 36, p. 34), &c.^ At the points of perforation the intine is generally thicker, often 
forming hemispherical protuberances which furnish the first material for the formation 
of the pollen-tube (Fig. 381, i), or the extine only forms thinner longitudinal striae 
which fold inwards when the pollen-grain becomes dry (as in Gladiolus, Yucca, 
Fl(j. 3S0.— Transverse section of a pollen-grain of F.pilo- 
biu7n atignstifoiiiun: a the points where the intine i pro- 
trudes, the intine being there thicker and the extine e thinner 
(Xsoo). 
Fig. 381.— Pollen-gram of AltJii. a rosea: A a piece of the 
extine seen from without ; B the half of a very thin section 
through the middle of the pollen-grain, st large spines, Ks 
small spine of the extine, o perforations through the extine e, 
i the intine,^ the protoplasm of the pollen-grain contracted 
{X800). 
Helleborus, &:c.). Very commonly however the intine is uniformly and continuously 
thickened, as in Canna, Sireliizia, Musa, Per sea, &c. ; and in this case, according to 
Schacht, no definite spots are prepared beforehand where the perforation is to take 
place. The number of these peculiarly organised points of perforation is definite in 
each species, often in whole genera and families ; there is only one in most Mono- 
cotyledons and a few Dicotyledons, two in Ficus, Justicia, &c., three in the Onagrarieae, 
Proteaceae, Cupuliferae, Geraniaceae, Compositae, and Boragineae ; four to six in 
Impatiens, AsirapcEa, Alnus, and Carpinus, while the number is large in Convolvulaceae, 
^ For more minute details see Schacht, Jahrb. für wissensch. Bot. II. p. 109, and Luerssen, 
ibid. VII. p. 34. — [Fritzsche, Beiträge zur Kenntniss des Pollen, Berlin, 1832. — Mohl, Beiträge zur 
Anatomie u. Physiologie der Gewächse, ist Heft, Bern, 1834. — Edgeworth, Pollen, London, 1877.] 
