POLLINATION AND FERTILISATION 265 



step is the germination of the pollen-grain, witli the formation 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, ^hich the tube conveys, with the female gamete or ovum. 

 This fusion is called Fertilisation. These several events must be con- 

 sidered separately and in their natural succession. 



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

 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 the mobility of animals is taken advantage of. The 

 mechanism of flowers has been specialised in the most remarkable 

 manner in accordance with these methods of transfer. Where use 

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

 easily shaken out in clouds from anthers often balanced on very 

 flexible filaments. The stigmas meanwhile arc much branched and 

 feathery, so as to expose a large surface for catching the grams. 1 hese 

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

 small and inconspicuous. Where animal agency is used, the llowcrs 



