n8 BOTANY 



As in each perfect flower both spores are produced, the 

 problem at first sight appears to involve only the transfer- 

 ence of its pollen grains to its stigma. In many cases this 

 is all that takes place, but simple as the method is, its 

 occurrence is the exception rather than the rule, for much 

 stronger and healthier plants -are yielded when the pollen 

 from one flower is made to fall upon the stigma of another. 

 The former simpler process is termed self-pollination; it 

 was probably the most primitive, but became gradually 

 superseded by the latter, known as cross-pollination. 



The necessity for pollination and the advantages 

 presented by the crossing have led to the development 

 of various mechanisms in flowers to bring it about. 

 Here, perhaps more than in any other case, do we 

 recognise the difficulties entailed by the stationary 

 situation of the plants. The small spores though set free 

 are non-motile and must be carried to the stigma by 

 some external agency which is not under the control of 

 either plant. Hence it is that we find perhaps more 

 adaptations or modifications of structure in connection 

 with this function than any other. 



The transport of the pollen at the outset must have 

 been brought about by the physical agents of nature, 

 water in the case of aquatic flowering plants, wind in 

 that of those of terrestrial habit. In both these cases the 

 chances of the liberated pollen being floated or blown to 

 the stigma are not great. Hence the pollen in such 

 flowers is always produced in large quantities. Aerial 

 transport is further aided by the production of very 

 light, dry, relatively smooth pollen, in some cases aided 

 by mechanical modifications of the wall, expansion 

 into bladders, etc. The stigmas in plants which are 

 pollinated in this way are often divided a good deal, 

 becoming in the case of the grasses quite feathery, so as 

 to offer as much surface as possible to entangle the 

 pollen. The surface of the stigma often bears a velvet- 



