XVIT] EFFECTS OF POLLEN 171 



living tissues. Petals which remain turgid and intact 

 for weeks if the flower is not pollinated may turn limp 

 and brown in a few hours if the pollen-tubes form 

 (Orchids, &c.), and in many cases (Oak, some Orchids, &c.) 

 the ovules do not grow at all until the pollen-tubes begin 

 their descent. All these, and similar phenomena, suggest 

 a stimulated flow of food-materials and water to the 

 centres of activity, and this flow may continue for long 

 afterwards to the excited organs, as evinced in the filling 

 out of the seeds and fruits. 



In the vast majority of plants the pollen-tube at 

 length reaches the micropyle of an ovule, a process 

 mechanically facilitated by the bringing of the micropyle 

 close to the base and placenta in anatropous and cam- 

 py lotropous ovules, and probably by chemotropic actions 

 where the tubes have to wander over the surface of 

 others. In Gymnosperms the pollen-grain germinates in 

 the orifice of the micropyle itself. 



Recent researches have shown, however, that in 

 Betulaceae, Hazel, Hornbeam, Casuarina, and Walnut, 

 the pollen-tube does not abandon the tissues of the style 

 for the cavity of the ovary, but grows down in the tissues 

 of the carpellary walls and enters the funicle and raphe, 

 passing into the base of the nucellus by the chalaza, and 

 thence penetrates the embryo-sac from below, and applies 

 its end to the oosphere. This mode of fertilisation has 

 been termed chalazogamic in contradistinction to poro- 

 gamic. 



