POLLEN. 



37 



afterwards brought into contact with the extremity of 

 the female organ. 



204. The origin of these grains of pollen seems to 

 be the following : as soon as the walls of some of the 

 cellules of the tissue of the anther have become con- 

 verted into a sort of consistent mucus, there form 

 within it a great number of little grains which serve as 

 nuclei, around which the mucus hardens to a membrane, 

 becoming in a little time developed in the form of a 

 spherical cell, within which remains the nucleus. Be- 

 tween the nucleus and the spherical cell is still con- 

 tained some mucus ; other nuclei are formed within 

 this, around which the mucus hardens to a membrane, 

 as before ; each of these nuclei is the origin of four 

 grains of pollen caused by the nucleus separating into 

 four distinct cells : the rest of the structure we have 

 spoken of becoming absorbed and destroyed by pressure 

 as the pollen grains increase in size. 



205. Generally all the pollen grains are free or dis- 

 tinct from each other, though they appear, in some 

 cases, connected by a great number of thready fibres, or 

 cobwebby-looking matter. 



206. There are plants, however, as the Orchidese, 

 in which the grains of pollen are united together by 

 fours, as at (fig. 37, a) ; this arises, according to the 



