204 



FORMS OF POLLEN-GRAINS. 



meter. Their form is much diversified. The most common form is 

 ellipsoidal (figs. 358, 359), more or less narrow at the extremities, 

 which are called its poles, in contradistinction to a line at e, equidistant 

 from either extremity, and which is its equator. In figs. 359, 360, 

 1 and 2, the two surfaces of the pollen-grains of Alh'um fistulo- 

 surn and Convolvulus tricolor are represented with their poles, p, 

 their equator, e, and the longitudinal folds in their membrane; 



360 



362 363 



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, triangular, trigonal (fig. 362), or polyhedral 

 figure (fig. 364). In the latter case, when there are markings on their 

 surface, those at the poles, p, sometimes differ from those at the equa- 

 tor, e. In Tradescantia virginica, the pollen is cylindrical, and becomes 

 curved ; it is polyhedral in Dipsacea? and Compositae ; nearly trian- 

 gular in Proteacese and Onagrariaceae. The surface of the pollen- 

 grain is either uniform and homogeneous, or it is marked by folds 

 dipping in towards the centre, and formed by thinnings of the mem- 

 brane. In Endogenous plants there is usually a single fold (fig. 359); 



Fig. 359. Pollen of Allium fistulosum. 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. 360. Pollen of Convolvulus tricolor. The letters and numbers have the same signification 

 as in fig. 359. 



Fig. 361. Grain of pollen of Cannabls sativa, or common Hemp. , Eqiiator. p p, Poles. 



Fig. 362. Pollen-grain of O3nothera biennis entire, with three angles, where tubes are pro- 

 duced. 



Fig. 363. The same, with one of its angles giving origin to a pollen-tube, which is formed by 

 the intine. When the tube protrudes, the extine is ruptured. 



fig. 364. Polyhedral pollen-grain of Cichorium Intybus, or Chiceory. 



