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 
