98 THE NATURE AND WORK OF PLANTS 



are more rigid than the living members. This is 

 due to the fact that even the soft living tissues 

 become hard when dead and dry. 



Stems secure firmness by the presence of hard 

 mechanical tissues and by filling soft tissues full of 

 water under pressure, as in leaves. 



128. Arrangement of hard or dead cells to secure 

 firmness of stems. — The dead cells of stems are 

 arranged in the form of strands or girders after the 

 principles used by an engineer or architect in con- 

 structing a tower or tall building. The architect 

 uses wood, brick, cement, and metal as material 

 from which to construct the tower. The plant uses 

 wood, bast, which resembles cable or wire rope in its 

 properties and is as strong as wroughl^iron, col- 

 lenchyma, which is elastic, and also soft pith cells. 

 The properties of wood are too well known to need 

 discussion. Bast cells make up the fibres which are 

 taken from the flax plant and used by man, and col- 

 lenchyma forms the sharp angles of the stems of the 

 mints and is very much like cartilage or rubber. The 

 plant has thus rigid beams, flexible cables, and soft 

 spongelike filling or cement for its building mate- 

 rials, and the towers it constructs are greater in 



