124 GENERAL BIOLOGY 



external or internal, to which the softer tissues are 

 attached or by which they are inclosed. This 

 skeleton, whether of chitin, or bone, or merely of 

 connective tissue, owes its rigidity to the deposit 

 among the living cells of a non-living intercellular 

 substance ; the cells themselves have very thin 

 walls. In plants, on the other hand, we find that 

 the intercellular substance is laid down in the form 

 of dense cell-walls of the cells themselves, and that 

 nearly the whole tissue of the stem or root is in one 

 sense skeleton. This intercellular substance is a 

 complex carbohydrate called cellulose, or a deriva- 

 tive of cellulose, lignin, and although it forms the 

 cell-walls it is, of course, not living substance itself, 

 any more than the plates of lime in bone. Not all 

 the cells are uniformly developed in this manner. 

 In most plant tissues there has arisen a differentiation 

 of the supporting or " mechanical " tissue which 

 frequently occurs in bundles or strands of fibers, 

 constituting a sort of internal skeleton. Hemp and 

 flax are abundantly supplied with these fibers, which 

 provide us with linen, hempen cord, etc. On the out- 

 side of roots and stems, particularly of the larger 

 plants (trees), the cell-walls become enormously 

 thickened, with an accompanying diminution of the 

 protoplasmic substance, to form bark or cork. 

 The cork is impervious to water and may be com- 

 pared with a secreted exoskeleton, like chitin, which 

 protects the softer living portion beneath. In trees 

 the whole central part of the stem is composed of 

 solid supporting tissue (wood), the living portion of 



