56 THE SOIL AND THE SAP 



Thus the carbohydrates, being soluble, diffuse down through 

 the cells and possibly through the vessels, while the proteins, 

 being insoluble products, must be conducted through tubes, 

 the sieve-tubes. Typical examples of storehouses are the 

 underground tubers which we call potatoes and the under- 

 ground stem of the bracken fern. 



The upper portions of a soft plant such as a stinging nettle 

 are kept erect by the turgidity of their cells. The great 

 majority of the cells are kept in a stretched condition by the 

 pressure of the watery spaces inside their protoplasm, pressing 

 it tightly against the cell wall. Should, however, the supply 

 of water from the roots be cut off, the plant waits, withers, 

 droops and loses its normal and healthy shape. But in those 

 plants which are not annual, which do not die down each 

 autumn and reappear each spring, in such plants as trees and 

 in the lower parts of most herbaceous plants, there is a strong 

 skeletal element made up of cells which have lost their proto- 

 plasm, and ^vhose cell walls have become immensely thickened 

 so that there is hardly any cavity left in the cell. Some of these 

 cells called fibres change the nature of their cell-wall into 

 woody substance or lignin. Such fibres are elongated dead 

 cells, interlocked so as to form a tough and hard skeletal 

 tissue. When mechanically "shredded out" they are used 

 in the making of paper and linen and certain coarse fabrics. 

 The fibres with other skeletal elements make the bodies of 

 woody plants much harder than the bodies of animals. If 

 we except those animals which are protected by a hard outer 

 skeleton, it is very much easier to hammer a nail into an 

 animal than into a tree. The arrows which pierced the body 

 of St Sebastian would not have penetrated the bark of a 

 cedar. 



