88 POLLEN : POLLINIA [CH. X 



Gucurbita, with spicate outgrowths and special apertures, 

 covered with lids, for the emergence of the pollen-tubes ; 

 the angular pollen-grains of Tropceolum, Dandelion, &c.; 

 the filamentous grains of Zostera ; the winged ones of 

 Pinus, Abies, and other Conifers, where the outer coat is 

 inflated on each side like a balloon, thus offering a large 

 surface to the wind. 



In some cases, e.g. Onagracese, the individual pollen- 

 grains do not separate completely into dry powder, but 

 remain loosely attached as if by thin filaments of cobweb ; 

 these filaments are drawn-out threads of a viscid substance 

 adhering to the grains. 



In other cases the pollen-grains never become isolated 

 at all, but occur in tetrads (e.g. Azaleas, Rhododendrons 

 and other Ericacea?, some Orchids, species of Juncus and 

 Luzula, Anona, Drimys, Jussieua, Apocynacece, Agave, 

 Typha, &c.), or in packets of 8, 16, or 32 grains closely 

 agglutinated, e.g. many Mimosese ; while in Orchids and 

 Asclepiads the whole mass of pollen-grains of each anther- 

 lobe remains thus unseparated. Such masses, which may 

 include hundreds of grains, are termed pollinia, and each 

 pollinium is removed by an insect owing to certain 

 mechanical adaptations for the purpose. 



