3] PHYSIOLOGICAL REACTIONS 87 



Fig. 21. — Features promoting aeration. A, Jussiaea repens, a rooting or floating 

 aquatic with numerous inflated roots which project upwards into the air and 

 contain a great development of air spaces through which air can pass to submerged 

 organs ( • \); B, Water-hyacinth {Eic/ilioinia crassipes), with leaf-stalks modified 

 for buoyancy, the whole plant floating freely ( ■, 5); C, cross section of leaf- 

 stalk of a Water-lily {Nymphaea steUata), showing large air-passages ( X 30). 



rents, and, having so migrated, often multiply to cover large areas 

 of water. 



Also significant in enabling plants to grow in many situations 

 where otherwise they could not exist, are modifications for climbing, 

 twining, scrambling, and running. Examples are shown in Fig. 22. 

 Further modifications apparently playing a similar role in plant 

 geography include those for catching insects to supplement the food 

 supply, and those for storing food to tide over the adverse period 

 of winter. Examples of carnivorous plants and of food-storage in 

 special underground organs are shown in Fig. 23 ; included in the 

 latter category are Potatoes and many bulbs and other structures 

 that are, besides, reproductive in function. 



The giving off of water-vapour from the aerial parts of plants 

 helps to keep them cool, and many are further protected from 

 intense sunlight by their structure or covering, so that ' scalding ' 

 and other injury may be averted even in very hot and sunny deserts. 

 The structural changes which restrict or accelerate the rate of water 

 loss are in general either hereditary and consequently characteristic 

 of the race, or are acquired by an individual plant or part of a plant 



