FORMS OF POLLEN-GRAINS. 205 



in Exogens, ofteii three (fig. 360). Two, four, six, and even twelve 

 folds are also met with. 



425. There are also pores or rounded portions of the membrane 

 visible in the pollen -grain. These vary in number from one to fifty. 

 In Endogeus, as in Grasses, there is often only 



one (fig. 365); while in Exogens, they number 



from three upwards. When numerous, the 



pores are either scattered irregularly (fig. 366), 



or in a regular order, frequently forming a 



circle round the equatorial surface (fig. 361). 



Sometimes at the place where the pores exist, 



the outer membrane, in place of being thin and 



transparent, is separated in the form of a lid, 



thus becoming operculate (operculum, a lid), as 



in the Passion-flower (fig. 354) and Gourd (fig. 



355). Grains of pollen have sometimes both 



folds and pores. There may be a single pore 



in each fold, either in the middle (fig. 367), or 



at the extremities; or folds with pores may 



alternate with others without pores; or finally, the pores and folds 



may be separate. 



426. The form of the pollen-grains is much altered by the applica- 

 tion of moisture. Thus, in fig. 367, 1, the pollen-grain of Lythrum 

 Salicaria, when dry, has an ellipsoidal form, but when swollen by the 

 application of water, it assumes a globular form (fig. 367, 2). This 

 change of form is due to endosmose, and depends on the fovilla being 

 denser than the water. If the grains are retained in water, the disten- 

 sion becomes so great as to rupture the extine irregularly if it is 

 homogenous, or to cause projections and final rupture at the folds or 

 pores when they exist. The intine, from its distensibility, is not so 

 liable to rupture, and it is often forced through the ruptured extine, 

 or through the pores, in the form of small sac-like projections (figs. 

 367, 2, 362). This effect is produced more fully by adding a little nitric 

 acid to the water. The internal membrane ultimately gives way, and 

 allows the granular fovilla to escape (fig. 357 /). If the fluid is ap- 

 plied only to one side of the pollen-grain, as when the pollen is applied 

 to the pistil, the distension goes on more slowly, and the intine is pro- 

 longed outwards like a hernia, and forms an elongated tube called a 

 pollen-tube (fig. 363). This tube, at its base, is often covered by the 



Fig. 36-3. Pollen -grain of Dactylis glomerata, or Cocks-foot grass. 



Fig. 366. Pollen-grain of Fumaria capreolata. 



Fig. 367. Grain of pollen of Lythrum Salicaria, showing six folds, three of which are per- 

 forated by a pore in their middle, "and three alternating with them have no pores, p p, Poles. 

 e e, Equator. 1. The grain in a dry state. 2. The grain swollen in water, so as to take a globular 

 form and display its folds. The intine or internal membrane begins to protrude through the 

 pores. 



