8o 



,KGANOGRAPHY. 



248 represents a pollen grain of the Mallow ; Fig. 249, that of a 

 Lily ; Fig. 250, that of Chicory ; Fig. 251, that of Evening Prim- 

 rose, and Fig. 252, that of the Pine. They are all considerably 

 magnified. 



When a pollen grain is placed upon the stigma of a flower of 

 the same species, the outer coat of the grain bursts, and by the 

 extension of the inner one, a tube is formed which penetrates the 

 tissues of the style. Fig. 253 represents a germinating pollen 

 grain. 



Pollen grains are produced in enormous num- 

 bers, particularly in plants that are dependent on 



Fig. 2 



Fig. 248. 

 Fig 249. 

 Fig. 250. 

 Fig. 251 

 Fig. 252 

 Fig. 253 



Fig. 249. 



Fig. 250. 



Fig. 251. 



Fig. 252. 



Fig. 253. 



— Spinose pollen grain of the Mallow. 

 — Pollen grain of Lily. 



— Pollen grain of Chicory. 



— Pollen grain of Evening Primrose. 



— Pollen grain of Pine. 



— Pollen grain in process of germination. 



All highly magnified. 



the wind for the transfer of the pollen from the stamens to the 

 pistils, as in the Pines. Some one has estimated that a single 

 plant of the Chinese Wisteria, when well developed, may pro- 

 duce during one flowering season as many 

 as 27,000,000 pollen grains. A common 

 Pine tree, in all probability, produces a 

 vastly greater number than this. 



It is to be observed that in plants that 

 are dependent on the wind for the transfer 

 of their pollen, the grains are usually dry 

 and powdery ; while those that depend on 

 insects for this work, usually produce sticky 

 pollen. In some instances, however, the 

 pollen does not separate into grains, but 

 remains in masses, as in the flowers of the 

 common Milk-weed and those of most Mllkweed 

 Orchidacese. These pollen masses are termed pollinia. 

 pollinia of the Milk-weed are illustrated in Fig. 254. 



Fig. 254. — Pollinia of the 



The 



