THE FLOWER 22] 
FERTILIZATION 
Growth of pollen tube. The transfer of the pollen from the 
anthers to the stigma is known as pollination and is followed 
by the growth of 
the pollen tube which 
leads to fertilization. 
After the pollen grain 
has been deposited on 
the stigma, it sends 
out a protuberance, or 
pollen tube (Fig. 226), 
which grows down 
through the style till 
it reaches the ovule. 
In passing through 
the style the tube 
absorbs nourishment 
from the cells of the 
style. After reaching 
the ovary the pollen 
tube, continuing to 
erow, enters an ovule 
(Fig. 4). 
There are three 
nuclei in the pollen 
tube. One of these is 
known as the tube 
nucleus and appears 
to govern the activity Fig. 226. Germination and growth of pollen 
of the pollen tube; grains on a stigma of the purslane (Portulaca 
the other two are male oleracea). (x 165) 
nuclei. Their func- 
tions will be better understood after we have considered the ovule. 
Structure of ovule. The ovule (Fig. 227) consists of a cen- 
tral mass, the nucellus, which is joined to the ovary wall by a 

