486 



PHANEROGAMS. 



when the pollen-grain becomes dry (as in Gladiolus, Yucca, Helleborus, &c.). Very 

 commonly however the intine is uniformly and continuously thickened, as in Canna, 

 Strelitzia, 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 peculiarly organised points of perforation is definite in each species, often 

 in whole genera and families ; there is only one in most Monocotyledons and a few 

 Dicotyledons, two in Ficus, Justicia, &c., three in the Onagrarieae, Proteaceae, Cupu- 

 liferae, Geraniacese, Compositae, and Borraginese; four to six in Impatiens, Astra- 

 pasa, Alnus, and Carpinus, while the number is large in Convolvulacese, Malvaceae, 

 Alsineae, &c. (see Schacht, I.e.). The extine is rarely smooth, more often marked 



JW;> 



Fig. 349.— Transverse section of a pollen-grain of Epilo- 

 binm angtisti/oliutn: a the points where the intine i pro- 

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



Fig. 350.— 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, A j 

 small spine of the extine, o perforations through the extine e, 

 t'the intine,/ the protoplasm of the pollen-grain contracted 

 (X800). 



on the outside by the sculpture to which reference has already been made. When 

 it is very thick, layers of different structure and texture may frequently be detected, 

 and differentiations sometimes occur in a radial direction, penetrating the thickness 

 of the extine (Fig. 350), and giving it in some cases the appearance of consisting of 

 rod-shaped prismatic pieces or of honeycomb-like lamellae, &c. These peculiarities 

 of structure recall those of the exospore of Marsileaceae, and probably only de- 

 pend, as in that case, on a further development of the radial striation, accompanied 

 possibly by subsequent absorption of the soft areolae and hardening of the denser 

 parts (see p. 30). The contents of the ripe pollen-grain, the Fovilla^ of the older 



^ [On the constitution of the ' amyloid corpuscles ' in the fovilla of pollen see Saccardo, Nuovo 

 Giornalc Botanico Italiauo, 1872, p. 241. — Ed.] 



