POLLINATION AND FERTILISATION 301 



same, or a corresponding flower. That transfer is called Pollination. 

 The second step is the germination of the pollen-grain, with the forma- 

 tion of a pollen-tube, which makes its way from the stigma to the 

 micropyle of the ovule, and conveys the contents of the pollen-grain or 

 microspore to the embryo-sac, or megaspore. The final step is the 

 fusion of the male gamete, which the tube conveys, with the female 

 gamete or ovum. This fusion is called Fertilisation. These several 

 events must be considered separately and in their natural succession. 



The distance through which the pollen-grain must travel from the 

 pollen-sac to the stigma varies greatly. In flowers such as the Butter- 

 cup, containing both stamens and carpels {hermaphrodite), the distance 

 may be small. But in many cases only stamens or carpels are produced 

 in the individual flower, and the grains must then be transferred from 

 one to the other. If the staminate and pistillate flowers are borne on 

 the same plant, the condition is described as diclinous, as in the Hazel, 

 Beech, or Oak. They may, however, be borne on distinct plants, 

 which are then styled dioecious, as in the Rose-Campion, or Willow. 

 These are examples of the separation of the pollen-sacs and ovules in 

 space. But there may also be separation of them in point of time. For 

 even where they are seated side by side the pollen-sacs may not shed 

 their pollen at the time when the stigma is ready to receive it. Two 

 possible cases exist. The stamens may shed their pollen before the 

 stigma of the same flower is fully matured, as in the Willow-herb, or in 

 the Compositae. This is the more common state, and it is described 

 as protandrous. Or the stigma may mature first, and be no longer 

 receptive when the stamens of the same flower shed their pollen. 

 This is the less common state, and it is seen in the Figwort and 

 Plantain. It is described as protogynous. Obviously the practical 

 effect is the same as the separation in space, for in either event the 

 pollen must be brought from elsewhere, if fertilisation is to succeed. 

 In such cases the distance to be traversed may be considerable, and 

 the plant has no means of its own for making the transfer. 



Use is then made of outside agencies, such as the movements of wind 

 or water : or advantage is taken of the mobility of animals. The 

 mechanism of flowers has been specialised in the most remarkable 

 manner in accordance with these methods of transfer. Where use 

 is made of wind the flowers produce abundance of dry dusty pollen, 

 easily shaken out in clouds from anthers often balanced on very 

 flexible filaments. The stigmas meanwhile are much branched and 

 feathery, so as to expose a large surface for catching the grains. These 

 features go with close grouping of the flowers, which are individually 



