232 



FOEMS OF POLLEN-GEAINS. 



poles, p, sometimes differ from those at the equator, e. In Tradescantia 

 virginica the pollen is cylindrical, and becomes curved; it is polyhedral 

 in Dipsacacese and Oompositse ; nearly triangular in Proteacese and 

 Onagracese (fig. 396). The surface of the pollen-grain is either uniform 





Fig. 396. 



Fig. 396. 



Pig. 397. 



Fig. 398. 



and homogeneous, or it is marked by folds dipping in towards the centre, 

 and formed by thinnings of the membrane. In Monocotyledonous 

 plants there is usually a single fold (fig. 393) ; in Dicotyledons, often, 

 three (fig. 394). Two, four, six, and even twelve folds are also met 

 with. 



There are also pores or rounded portions of the membrane visible 

 in the pollen-grain. These vary in number 

 from one to fifty. In Monocotyledons, as in 

 Grasses, there is often only one (fig. 399) ; 

 while in Dicotyledons, they number from 

 three upwards. When numerous, the pores 

 are either scattered irregularly (fig. 400), or 

 in a regular order, frequently forming a circle 

 round the equatorial surface (fig. 395). Some- 

 times at the place where the pqres 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. 388) and 

 Gourd (fig. 389). Grains of pollen have 

 sometimes both folds and pores. There may be a single pore in 

 each fold, either in the middle (fig. 401) or at the extremities ; or 



Fig. 395. Grain of pollen of Cannabis sativa, or coininon Hemp, e, Equator. j);p, Poles. 

 Fig. 396. Pollen-grain of (Enothera biennis, entire, with three angles, where tubes are pro- 

 duced. Fig, 397. The same, with one of its angles giving origin to a pollen-tube, which is 

 formed by the intine. When the tube protnides, the extine is ruptured. Fig. 398. Poly- 

 hedral pollen-grain of Cichorium Intybus, or Chicory. Fig. 399. Pollen-grain of Dactylis 

 glomerata, or Cocks-foot grass. Fig. 400. PoUeu-grain of Furaaria capreolata. Fig. 401. 

 Grain of pollen of Lythrum Salicaria, showing six folds, three of which are perforated by 

 a pore in their middle, and three alternating with them have no pores ; p p, poles ; e c, 

 equator. 1. The grain in a Avj state, 2, The grain swollen in water, so as to take a globu- 

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

 the pores. 



Fig. 401. 



