Chapter 5. 
FLOWERS AND THEIR FORMATION. 
Conditions favouring the production of flowers. Structure and 
functions of the various parts of a flower. Pollination and 
. Fertilisation. Self-fertile and self-sterile flowers. Ripening of 
fruits and seeds. 
In speaking of the difference between vegetative and 
reproductive organs, in the first chapter I have already 
mentioned that certain external conditions which favour 
the development of the former affect adversely the forma- 
tion of flowers. Long horticultural practice has also 
proved that the development of floral organs can be 
stimulated in various ways which are well known to 
gardeners. Thus reduction in the supply of water and 
consequent stoppage of the luxuriant development of 
leaves is one of the chief methods employed. It has been 
found by scientific experiments that these various horti- 
cultural practices are based upon definite physiological 
requirements. In recent years much has been done to 
confirm and extend our knowledge of this subject. It 
was proved that bright illumination of plants is essential 
for the production of flowers. At first it was thought 
that certain rays of light influenced the development. in 
the plant of special flower-forming substances, and early 
experiments seemed to indicate that these were produced 
by definite rays of light from beyond the blue end of the 
spectrum—rays of light which are known to have great 
chemical activity. More recently, however, it seems to 
22 
