wi.) THE FERTILISATION OF FLOWERS. 35 
unisexual. If both are found in the same flower, it 
is bisexual. Sometimes, again, the staminate flowers 
SF. 
if 
PRS 
we Ov 
a 
Fic. 41. Fic. 42. Fic. 43. 
are produced on one plant, the pistillate on another. 
In this case they are known as diecious plants. If 
both are found on the same plant, they are mov- 
acious. To effect fertilisation, some of this pollen 
must be conveyed from the anther to the stigma. 
Supposing a pollen grain adheres to the stigma, 
the moisture of that organ induces it to send 
out from its under side a very slender 
shoot, the pollen tube, which pierces 
the stigma and, increasing in length, 
penetrates the whole length of the style, 
finally entering the ovary, where it 
comes in contact with an ovule. As the 
result of this contact, the protoplasm of 
the pollen tube mingles with that of the ovule, cell- 
division takes place, and the seed gradually ripens. 
But before this can take place, certain difficulties 
must be overcome. If the pollen-bearing (staminate) 
flowers are on one plant, and the pistillate flowers on 
another, how is the pollen to be placed on the stigma ? 
Again, in those flowers which possess both stamens 
and pistil, the stamens open and discharge their 
