POLLEN ON THE STIGMA. 403 



It has been already mentioned (p. 156) that the flowers of the American Yuccas 

 are pollinated by small moths, unknown in Europe, and that these flowers set no 

 fruit with us. Also, that several species of Catchfly, indigenous to the Southern 

 and Eastern Alps, though robbed of their honey by humble-bees, are but seldom 

 pollinated and fruitful (p. 239). 



Lastly, many cultivated plants, depending on insects for fertilization, flower in 

 gardens earlier or later than in their wild surroundings. In nature, their period of 

 flowering coincides with the time of flying of cei'tain insects; in gardens — even 

 when the insects in question occur — the flowers may be too early or too late for the 

 insect season. From what has been said above, it will appear that sterility in many 

 cases is only apparent, and is due merely to the lack of the normal and necessary 

 conditions for bringing about pollination. 



In other cases it may happen that although the stigmas are pollinated, the pollen 

 cells are aboi'tive and incapable of producing pollen-tubes. This condition of the 

 pollen obtains most frequently in gardens amongst plants on rich, well-manured 

 soil, artificially-produced hybrids, and plants whose stamens are partly converted 

 into petals. At the same time it must not be supposed that by any means all plants 

 which show a "doubling" produce bad pollen; for instance, many double Roses pro- 

 duce pollen which is used by gardeners for artificial pollination with good i-esults. 

 Still, in the majority of such plants abortive pollen is the rule, pollen which is non- 

 eflective on the stigma. 



In nature, especially in places where many plants flower at the same time, as on 

 the borders of a wood, in meadows and heaths, it is unavoidable that the pollen of 

 various species should be deposited on one and the same stigma. Insects certainly 

 show a prefei'ence for a single species for considerable periods, particularly when 

 this species is flowering in quantity on a confined space; still, anyone who closely 

 observes insects visiting flowers can easily convince himself that the flowers visited 

 are changed from time to time. A bee which has just dusted itself with pollen in 

 the flower of a Winter Aconite {Eranthis) will fly across to visit a bush of Scdix 

 daphnoides, and as it passes a plant of Daphne Mezerewm it will suck its honey; a 

 moment later it will swoop down to the flowers of Crocus vernus in the meadow 

 near by, and then fly on to the Sweet Violet (Viola odorata). On the stigma of the 

 last-mentioned plant will be found the pollen of all or several of the just-visited 

 flowers, on the Crocus that of the Willow, and so on. The case is similar with 

 wind-pollinated flowers. I have found the pollen of the Spruce Fir {Ahies excelsa) 

 and of Dog's Mercury (Mercurialis perennis), both brought by the wind on to the 

 stigmas of the Herb Paris {Paris quadrifolia); and on another occasion the stigmas 

 of Gagea lutea were so thickly dusted with the pollen of Alnus viridis that there 

 would have been no room for any pollen more. 



That the pollen of the Willow will fertilize the Ci'ocus, that of Dog's Mercury 

 the Herb Paris, or that of the Alder the Gagea is a priori improbable. Only such 

 changes take place in the pollen as always occur when it is placed on a moist 

 substratum; all further stages in its development are arrested. The commencing 



