CAUSES OF STERILITY. 241 



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 

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

 not ejecting it with force, unless the filaments are irritable 

 enough to knock the anther violently against the pistil ; or, 

 unless the stigmatic apparatus possesses special irritability, as 

 is the case with certain Orchids. If we watch the Hazel, or 

 any of the Coniferous order, in which the enormous quantity 

 of pollen employed to secure the impregnation of the seed 

 renders it easy to see what happens, it will be found that am 

 pollen is scattered in damp cold weather ; but, in a sunny, 

 warm, dry morniug, the atmosphere surrounding such plants 

 is, in the impregnating season, filled with grains of pollen 

 discharged 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 ceUs tiU the power of impregnation is lost ; or perhaps 

 because, as a critic has suggested, the wet operates injuriously 

 upon the very constitution of the pollen, and of the stigmatic 

 surface. 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 

 Cucumbers, from which insects are excluded, no seed is formed 

 unless the poUen is conveyed by hand, from those flowers in 

 which it is formed, to others in which the young fruit alone is 

 generated. In all cases of this kind, the remedy for sterility, 

 where plants exist in an artificial condition, is evidently to set, 

 or fertilise them by hand; but, when they occur in the 

 orchard or the flower-garden, science suggests no assistance. 

 It is by hand-setting alone that in hot-houses, and in tropical 

 Asia, the VaniUa, a native of tropical America, can be 

 made to bear fruit ; because, as is believed, the insects which 

 haunt the Vanilla flowers in America, and set them, are 

 unknown in Europe and Asia. 



