ESSENTIAL ORGANS — POLLEN. 



231 



and that is said to be the intine. The extine is a firm membrane, 

 which defines the figure of the pollen-grain, and gives colour to it. It 

 is either smooth, or covered with numerous projections, granules, points 

 minute hairs, or crested reticulations (fig. 390). The colour is generally 

 yellow, and the surface is often covered with a viscid or oily matter. 

 The intine is uniform in difierent kinds of pollen, thin and transparent^ 

 and possesses great power of extension. It is said to be the first 

 envelope formed, the other being subsequently deposited while enclosed 

 in the parent cell. 



Within these coverings a granular 

 semifluid matter called fovilla is con- 

 tained, along with some oily particles, 

 and occasionally starch. The fovilla 

 contains small spherical granules, some- 

 times the sTshsir of ^1 inch in diameter 

 (fig. 391), and larger ellipsoidal or 

 elongated corpuscles (fig. 392), which 

 exhibit molecular movements under the microscope. 



Pollen-grains vary from ^^ to ^^ of an inch or less in diameter. 

 Their forms are various. The most common form of grain is ellip- 

 soidal (figs. 392, 393), more or less narrow at the extremities, which 

 are called its poles, in contradistinction to a line equidistant from 



Fig. 392. 



Fig. 393. 



Fig. 394. 



either extremity, and which is its equator. In figs. 393, 394, 1 and 

 2, the two surfaces of the pollen-grains of Allium fistulosum and 

 Convolvulus tricolor are represented with their poles, p, their equator, 

 e, and the longitudinal folds in their membrane ; while at 3 are shown 

 transverse sections at the equators, with a single fold in one case, and 

 three folds in the other. Pollen-grains are also of a spherical, tri- 

 angular, trigonal (fig. 396), or polyhedral figure (fig. 398). In the 

 latter case, when there are markings on their surface, those at the 



Fig. 391. Pollen-grain of Amygdalus nana, the intine or internal membrane of which is 

 protruding at three porefi, under the form of as many ampullae or sacs, ttt. One of these is 

 open a£ the extremity, and from it is discharged the fovilla, /, composed of variously-sized 

 granules. Pig. 392. Large granules of fovilla of Hibiscus palustris. Fig. 393. Pollen of 

 Allium flstulosum, p, Pole, e, Equator. 1. Pollen-grain seen on the face. 2. On the 

 opposite side or back. 3. Transverse section through its equatorial line. Fig. 394. Pollen 

 of Convolvulus tricolor. The letters and numbers have the same signification as in fig. 



