in.] THE FERTILISATION OF FLOWERS. 



35 



unisexual. If both are found in the same flower, it 

 is bisexual. Sometimes, again, the staminate flowers 



FIG. 



FIG. 41. 



"PS 



are produced on one plant, the pistillate on another. 

 In this case they are known as dioecious plants. If 

 both are found on the same plant, they are mon- 

 oecious. 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 



FIG. 44. 



