CHAPTER V 



THE STRAINS AND STRESSES IN TISSUES l 



SECTION 18. General. 



THE existence of strains in the tissues is shown by the curvatures and 

 changes of shape when particular tissues are wholly or partially isolated. 

 If, for example, the epidermis with a portion of the subjacent tissue is 

 removed in long tangential slices from a young petiole of Rheum or 

 Begonia, or from a young stem of Helianthus or Polygonum, these strips 

 shorten by from I to 4 per cent, of their length, while the central 

 cylinder elongates by from 2 to 6 per cent. The latter was, therefore, 

 compressed in the intact shoot, whereas the outermost tissues were 

 stretched and under tension. The length of the intact shoot represents 



FiG. 14. Median slices from a nearly adult internode of 

 Coleus Blumei, A, halved medianly ; B, sliced peripherally. 



FiG. 15. Section of a three- 

 year-old branch of Salix 

 Caprea, from which the bark 

 (r) has been removed and then 

 reapplied. 



the balance between the tendency of the epidermis to contract and of 

 the central cylinder to expand. Hence, if a longitudinal slice from the 

 centre of the stem is cut vertically into halves, these curve apart (cf. 

 Fig. 14). Similarly, longitudinal strips from the inflorescence axis of 

 Taraxacum Dens-Leonis curve outwards, and this bending continues in 

 water until they are spirally coiled. These curvatures afford a general 

 indication of the tissue-strains, but the absolute values can only be 

 determined by finding the weight, as tension or pressure per unit area, 

 necessary to restore the isolated tissue to its original length. 



Transverse or tangential strains also exist, and these become very 

 pronounced during secondary growth, for the increase in the diameter of 



1 [Throughout the following chapter this and similar expressions are given in their physical 

 terms, although direct equivalents of the vague German words are current in botanical physiology. 

 To speak of a tissue-tension when we often mean a tissue-compression, and in general a tissue-strain, 

 is absurd, and the use of the terms negative tension and positive tension for tension and compression 

 is highly confusing, and should be abandoned. A force producing a change of shape (strain) but no 

 alteration of volume is a shearing stress .] 



