37 
effects the pollination, which is the necessary preliminary 
to the fer/zlisation 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 poilen producing stamens and the ovule con- 
taining seed vessel, self-pollination does not usuallv 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 does 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 
lowers. 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, 9/-.) 
