100 POLLEN. 



A very conspicuous feature of many pollen-grains is the infinitely varied 

 sculpturing, &c., of their walls. Sometimes this takes the form of a delicate dotting 

 of the wall, as in Asarum, Meadow Saffron, Rue, Salvia, Gentians, and Euphorbias, 

 many Aroids and Musaceae (cf. figs. 217 13 and 217 H ); or the projecting ridges may 

 be transversely striated as in Saxifraga aizoides; or, again, delicate striations may 

 run in meridian-like circles (e.g. Brugmansia arborea). Sometimes dotted lines are 

 found arranged in various ornamental reticulating patterns. On the smooth surface 

 of the grains of Thesium alpinum and rostratum reticulations occur, and in the 

 centre of each mesh a distinct dot. Similiarly in Thrift and Sea Lavender 

 (Armeria and Statice), and in the Corn Cockle (Agrostemma Oithago). Often the 

 surface presents considerable unevenness. In Cuphea platycentra the outer coat is 

 prettily ridged, whilst in many other cases it is finely granulated. The little 

 projecting granules may be either scattered equally over the whole surface, or they 

 may be arranged in networks which is specially the case in Cruciferae (Capsetta, 

 Eaphanus, Sinapis). In the Passion Flowers (e.g. Passiflora Kermesina, fig. 217 4 ) 

 these networks are inclosed in shallow, ring-like depressions, whilst in Cobcea 

 scandens (fig. 21 7 x ) the surface has a honey-combed appearance. Sometimes the 

 whole surface is dotted over with little wart-like projections, as in Centaurea Jacea, 

 Mistletoe (Viscum album), White Water Lily (Nymphcea alba), and the tropical 

 Bauhinias (Bauhinia armata, furcata, cf. figs. 218 1 and 218 2 ); or it may be covered 

 with sharp, needle-like prickles, as in the pollen-grains of Composites, Scabiouses, 

 Campanulas, Cucurbitaceae, Malvaceae; also in the genera Armeria, Amaryllis, 

 Canna, Lonicera, Ipomcea, and Convolvulus (cf. figs. 21 8 3 and 21 8 9 ). 



It is only the superficial layer of the pollen-grain which shows these sculpturings 

 and projections, the inner layer, which abuts immediately upon the protoplasm, is 

 homogeneous. The wall of pollen-grains is, as a rule, three-layered. These three 

 layers are: the internal one or intine, the middle one or extine, and the external 

 one or perine. The extine and intine arise from the protoplasm of the pollen-cell 

 itself; the perine, on the other hand, is deposited from the matrix in which the 

 young pollen-grains lie embedded. It comes about in this way. The young grains 

 first clothe themselves with delicate walls, which are in due time thickened. This 

 is the extine. Within this they form a second layer, the intine. Lastly, the perine 

 is deposited upon them from without. The intine and extine can generally be 

 readily distinguished as separate layers, but between the extine and perine the 

 boundary is by no means so well marked. The various sculpturings, prickles, and 

 other unevennesses of the outer coat really appertain to the perine. 



It sometimes happens at definite spots on the wall of a pollen-grain, from a 

 separation of the molecules there, that little spaces or actual canals arise which open 

 externally by tiny pores. This may be well observed in Thesium, Prunella, 

 Ipomcea, and Gentiana. In these canals a yellow (rarely colourless) oil is con- 

 tained, which oozes out in the form of minute drops when the grains are moistened 

 and absorb water. Such at any rate is the behaviour in Prunella grandiflora and 

 Gentiana ciliata. In many other plants the whole surface of the grain is saturated 



