STRUCTURE OF THE POLLEN. 261 
The itine is the first formed layer, and appears to be of the 
same nature and appearance in all pollen-cells. It is usually 
smooth, very delicate, and transparent, and is composed of pure 
cellulose. {tis generally applied so as to form a complete lining 
to the extine, except perhaps in those cases where the latter 
presents various processes, as in (Hnothera, when Henfrey be- 
leves that the intine does not extend into them in the mature 
ollen. 
, The extine is a hard thick resisting layer, forming a kind of 
cuticle over the intine or proper cell-coat. While the intine 
usually presents a similar appearance in the pollen of different 
plants, the extine is liable to great variation ; thus it is some- 
times smooth, at others marked with little granular processes 
(fig. 73), or spiny protuberances (fig. 567), or reticulations (fig. 
571). The nature of these markings is always the same for the 
pollen of any particular species or variety of plant, but varies 
much in that of different plants. The extine is generally 
Fie. 567. Fic. 568. Fic. 569. Fie. 570. 
Fig. 567. Pollen of Hollyhock (Althea rosea). Fig. 568. Elliptical pollen 
of Milkwort (Polygala). e. Extine. f. Slits. Fig. 569. The same pollen 
viewed from above.— Fg. 570. Pollen-cell of Dactylis glomerata. After 
Jussieu, 
covered by a viscid or oily secretion, which is commonly sup- 
posed to be derived from matter remaining from the solution 
or absorption of the walls of the parent-cells. The colour of 
pollen-cells also resides in the extine. In by far the majority 
of cases the pollen-cells are yellow, but various other colours are 
also occasionally found; thus they are red in species of Ver- 
bascum, blue in some species of Hpilobiwm, black in the Tulip, 
rarely green, and occasionally of a whitish tint. 
Besides the various markings just described as existing on the 
extine, we find also either pores (fig. 570), or slits (figs. 568, f, 
and 569, f), or both pores and slits, and which vary in number 
and arrangement in different plants. At the spots where these 
slits or pores are found, it is generally considered that the extine 
is absent ; but some botanists believe that the outer membrane 
always exists, but that it is much thinner at these points than 
elsewhere. In the greater number of Monocotyledons there is 
but one slit ; while three is a common number in Dicotyledons. 
Sometimes there are six, rarely four, still more rarely two, and 
