FLOWER AND FRUIT 155 
Dandelion and oxalis flowers, for instance, close when 
removed to a dark place, no matter what may be the 
temperature or what the time of day. This affords 
the pollen protection from the dews and frosts. The 
opening of a flower bud is due to more rapid growth 
on the inside than on the outside of the petal base. 
Temporary opening and closing are produced by 
greater turgidity of the cells on the inner and outer 
sides respectively. 
Essential Parts. We now come to the essential 
parts of the flower to which all others are subsidiary. 
Even the vegetative organs of the plant—root, leaves, 
and stem—may be regarded as merely providing a 
means of producing and supporting these. The stamens 
and ecarpels alone are directly concerned in the actual 
work of reproduction, and it is to secure the proper 
functioning of these that the whole effort of the plant 
is concentrated. It is these that the sepals protect, it 
is to these that the petals attract. 
The Stamens collectively form the androecium 
(Gk. andros a man and ovtkos a house). They 
vary in number far more than the members of 
the first two whorls. They are usually free, as in the 
buttercup and rose, though in a few flowers, such as 
those of the bean and gorse, some or all of them are 
united. The free condition is the more primitive. Hach 
stamen, as a rule, consists of two parts, the filament 
or stalk and the little knob or anther at the tip. The 
anther generally has two lobes separated by a vertical 
depression. Each of these lobes is a pollen sac which 
eventually opens or dehisces to set free the little grains 
of pollen that are essential to the development of 
fruit and seed. Where the anther opens towards the 
interior of the flower, as in the case of the buttercup, 
the dehiscence is said to be introrse, and where it opens 
towards the outside, as in the iris, it is termed extrorse. 
