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 

 (Fio- 35> P- 33)' ^^ 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, /), or the extine only forms thinner longitudinal striae 

 which fold inwards when the pollen-grain becomes dry (as in Gladiolus, Yucca^ 



^d^^. 



riG. 380. — Transverse section of a pollen-grain of Ej>ilo- 

 btum angustifoHum: a the points where the intine i pro- 

 trudes, the intine being there thicker and tlie extine e thinner 

 (Xsoo). 



Fig. 381.— Pollen-grain of Althaa 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, Sirelitzia, Musa, Persea, &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 pecuHarly 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, Geraniacese, Compositae, and Boragineae ; four to six in 

 Impatiens, Asirapaa, Alnus, and Carpinus, while the number is large in Convolvulacese, 



^ For more minute details see Schacht, Jahrb. fiir wissensch. Bot. II. p. 109, and Luerssen, 

 ibid. VII. p. 34. — [Fritzsche, Beitrage zur Kenntniss des Pollen. Berlin, 1832. — Mohl, Beitrage zur 

 Anatomic u. Physiologic der Gewachse, ist Heft, Bern, 1834. — Edgeworth, Pollen, London, 1877.] 



