37 



effects the -pollination, which is the necessary preliminary 

 to the fertilisation of the flower. The pollen grains left 

 on the sticky surface of the stigma germinate there, and 

 a delicate tube grows down from them to the immature 

 seeds, which become fertilised by the fusion of the vital 

 element of the pollen grain with that of the ovule. Even 

 though a flower may produce both of these essential 

 organs, the pollen producing stamens and the ovule con- 

 taining seed vessel, self-pollination does not usually take 

 place as the stamens and seed vessel mature at different 

 times. This can easily be seen in such a large flower as 

 that of the Nasturtium, in which the stamens may be 

 observed ripening one after the other and liberating their 

 pollen, and only after the last of the pollen sacs has shed 

 its pollen doe^ the three-pronged stigma open and is then 

 ready to receive pollen brought from another flower in 

 which the stamens are opening. All the various and 

 wonderful mechanisms, by means of which flowers ensure 

 their pollination, have the purpose of securing the ferti- 

 lisation of the ovules with pollen from another flower and 

 if possible from another plant, for as Darwin has shown * 

 cross fertilisation usually causes the production of more 

 numerous and stronger offspring than self -fertilisation. 



Gardeners who grow exotic plants, the flowers of which 

 are adapted to the visits of insects not found in this 

 country, have often to perform this service themselves and 

 to pollinate artificially the flowers using usually a fine 

 camel-hair brush. In plants like the Tomato, tapping the 

 stems with a stick, wrapped round with a cloth to prevent 

 injury to the plant, will generally cause the pollen to drop 

 out of the opened anthers on to the stigma of these pendant 

 flowers. If tomatoes are grown under glass, we must take 

 care that during the period of flowering the house is kept 

 dry, as the pollen sacs will not open in a moist atmosphere 

 and the plants cannot be pollinated. Of course by the 

 method described self-pollination only can be effected, 

 crossing can be obtained by using a fine paint brush and 

 pushing the hairs up between the stamens first of the 

 flower and then of another. 



In some plants, such as the Cucumber and Marrow, the 

 flowers are of two kinds, some producing the seed vessel, 



Darwin, C. " The Effects of Cross and Self-Fertilization in the 

 Vegetable Kingdom." (Murray, Q/-.) 



