172 APPLICATION OP PRINCIPLES. 



the pollen of another flower. (See Sweet in the Gar- 

 dener's Magazine^ vii. 206.) 



An unfavourable state of the atmosphere obstructs 

 the action of pollen, and thus produces sterility. 

 Pollen will not produce its impregnating tubes in too 

 low a temperature, or when the air is charged with 

 moisture ; neither, in the absence of wind or insects, 

 have some plants the power of conveying the pollen 

 to the stigma, their anthers having no special irrita- 

 bility, and only opening for the discharge of the pollen, 

 not ejecting it with force. If we watch the Hazel, or 

 any of the Coniferous order, in which the enormous 

 quantity of pollen employed to secure the impregna- 

 tion of the seed renders it easy to see what happens, 

 it will be found that no pollen is scattered in damp 

 cold weather ; but, in a sunny, warm, dry morning, 

 the atmosphere surrounding such plants is, in the im- 

 pregnating season, filled with grains of pollen dis- 

 charged by the anthers. In wet springs the crops of 

 fruit fail, because the anthers are not sufficiently 

 dried to shrivel and discharge their contents, which 

 remain locked up in the anther cells till the power of 

 impregnation is lost. In vineries and forcing-houses 

 generally, into which no air is admitted to disturb the 

 foliage, nor any artificial means employed for the 

 same end, and when the season is too early for the 

 presence of bees, flies, and other insects, the grapes 

 will not set : and in the frames of melons and cucum- 

 bers, from which insects are excluded, no seed is 

 formed unless the pollen is conveyed by hand, from 

 those flowers in which it is formed, to others in which 



