BIOLOGY OF THE FLOWER 203 



attractive in colour and secrete honey, but, failing insect- 

 visits, self pollination commonly occurs, an advantage in 

 early flowering species.. In the Iris the stamens are peta- 

 loid and large enough to be conspicuous. The greatest 

 specialization occurs in the Orchids, which have irregular 

 flowers, long honey-secreting spurs, stamens reduced to one, 

 rarely two, and united to the style (gynandrous). They 

 are usually incapable of self-pollination. The capsules 

 produce an immense number of minute seeds. 



CHAPTER XV 



POLLINATION, FERTILIZATION, AND THE 

 ORIGIN OF SEEDS 



From our study of flowers we learn that all the parts 

 of which they are composed serve directly or indirectly to 

 secure the production of seeds. The modifications are very 

 numerous, but in most cases they are definitely related 

 to pollination, which precedes fertilization. Except in rare 

 cases, ovules must be fertilized before seeds can be developed. 



Advantages of self- and cross-pollination. The flowers of 

 most plants are developed in air, and the pollen-grains 

 have to be carried through air from the anther to the stigma. 

 The chief means by which this is secured may be sum- 

 marized as follows : 



1. Self-pollination, where the pollen falls on to the stigma 

 of the same flower. This occurs in flowers which never 

 open, like some of the Violets and Wood Sorrel, also in 

 certain species of Deadnettle and Vetch. These are known 

 as cleistogamic flowers (see p. 186). Self-pollination is 

 very common in flowers where the pollen and stigma are 

 ripe at the same time, e. g. in the Buttercup, Dwarf Mallow, 



