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CHAPTER I 



WATER PLANTS AS A BIOLOGICAL GROUP 



(i) INTRODUCTION 



WE are living at the present day in what may be described 

 botanically as the Epoch of Angiosperms, or Flowering 

 Plants. The members of this group now represent the dominant 

 type of vegetation and are distributed over nearly all the land 

 surfaces of the globe. The vast majority are typically terrestrial, 

 carrying^ their existence with their flowers and leafy shoots in 

 the air, but with their roots embedded in soil of varying degrees 

 of moisture, from which they derive their water supply. This 

 water supply is one of the prime necessities of their life, and in 

 their relation thereto, the plasticity of their organisation is 

 notably exhibited. At one end of the scale there are plants which 

 can withstand long periods of drought and are capable of flou- 

 rishing under desert conditions in which the water supply is 

 minimal. At the other extreme we meet with hydrophytes 

 plants which have exchanged terrestrial for aquatic life. Those 

 which have embraced this change most thoroughly, live with 

 their leafy shoots completely submerged, and have, in some 

 cases, ceased to take root in the substratum, so that all their 

 vegetative life is passed floating freely in the water which is to 

 them what atmosphere and soil are to terrestrial plants. The 

 ultimate term in the acceptance of aquatic conditions is reached 

 in certain hydrophytes with submerged flowers, in which even 

 the pollination is aquatic water replacing air as the medium 

 through which the pollen grain is transferred to the stigma. 

 These fundamental changes in habit are necessarily associated 

 with marked divergences from the structure and life-history of 

 land plants. The result has been that the aquatic flowering 

 plants have come to form a distinct assemblage, varying widely 



