254 PLANT-LIFE 



because the cells are turgid. An insufficient supply is 

 quickly demonstrated by limp and drooping leaves — a 

 phenomenon due to the fact that the cells are not 

 turgid. 



Owing to different conditions of the water-supply, 

 there are marked differences between land and water 

 plants. A true aquatic lives and moves and has its 

 being in water. It has no need to develop special means 

 for water absorption, because it can imbibe all it requires 

 through its general surface. It is for this reason that a 

 Seaweed has no root. Water and nutrient salts in solu- 

 tion are absorbed through its surface, and even the 

 carbon dioxide and the air that it requires are present 

 in the water. Hence an aquatic plant has no stomata. 

 Water plants have little need for a vascular system, 

 either for conduction or for support. They are sup- 

 ported by the water, and vascular bundles are either 

 poorly developed in them or non-existent. Certain 

 lowly land plants, which are not far removed from 

 aquatics, and which favour moist and shady situations, 

 can also absorb water through their general surface. 

 Liverworts are examples. 



We opine that land plants have evolved from water 

 plants, and in previous inquiries we have considered 

 the advance in structure demanded in the conquest of 

 the land; but it has happened in the great struggle for 

 existence that land plants have been jostled into water, 

 and have learned how to make themselves at home 

 there. We may assume that all flowering aquatics are 

 descended from land plants which accommodated them- 

 selves to the demands of an aquatic environment. 

 Among such are the Water-Lilies, whose leaves are long- 



