MECHANICAL PROBLEMS 91 



matter of engineering construction the plant 

 has nothing to learn of man although the 

 converse might not be equally true. The 

 more closely one examines the construction 

 of a plant from the mechanical point of view, 

 the more wonderful and complete does it 

 appear. Certain cells or cell tissues become 

 differentiated from their neighbours, and 

 develop the requisite strength, elasticity, 

 and other desirable qualities. They are not 

 distributed in the plant at haphazard, but 

 occur in situations where they are mechanic- 

 ally effective and physiologically appropriate. 

 Furthermore, they are united so as to form 

 definite tissue systems, and in connection 

 with the more specialised types we may, with- 

 out any exaggeration, speak of a mechanical 

 arrangement of tissues. 



Suitable rigidity is secured by the young 

 undifferentiated parts of plants in the same 

 way as in the more primitive ones, namely 

 by the pressure of the watery sap contained 

 within the cells. This confers the same 

 sort of resilience as an inflated rubber ball 

 possesses, and it amply suffices for many 

 aquatic forms, although it is not sufficient 

 for the needs of land plants generally, and 

 only serves for small species growing under 

 special conditions. An ordinary rooted plant 

 has not only to hold itself in position, but it 

 has to be capable of withstanding the effects 

 of forces that are repeatedly acting upon it. 

 Every time the wind blows, demands are 



