DEVELOPMENT AND ALTERATION OF THE TISSUE-STRAINS 65 



SECTION 19. The Development and Alteration of the Tissue-strains. 



It has already been mentioned that the tissue-strains result from the 

 unequal activities of growth of different cells and tissues. These strains 

 can only become pronounced when cells or tissues are formed, which offer 

 considerable resistance to the longitudinal or radial expansion of more 

 rapidly growing ones. Hence, in the primary meristems of stems and roots, 

 and generally in plastic tissues, no pronounced strains are produced. 

 The formation of relatively rigid tissues, such as the vascular bundles 

 and sclerenchyma which ultimately cease to grow, renders possible the 

 production of high internal strains, for these tissues may offer sufficient 

 resistance to the expansion of those still capable of growth as to produce 

 an enforced cessation of the latter function. 



The strains thus produced usually undergo further changes, due to 

 alterations of turgor, to the death of certain tissues, to thickening of the 

 cell-walls and changes in their elasticity, as well as to internal processes of 

 growth. Thus the compression disappears from the pith as it dies, and 

 when stems become hollow owing to the rupture of the pith, the original 

 compression has passed into a strong tension. Again, the transverse 

 tensions increase when secondary thickening begins, and, more especially 

 in roots, the previous longitudinal tensions in the wood and cortex become 

 reversed at this time. 



In some cases, as for example in the staminal filaments of Cynareae, 

 and in the pulvini of Mimosa and Phaseolus, the stresses and strains pro- 

 duced during development persist during the entire life of the organ. In 

 most cases, however, the tissues of an adult organ undergo less change of 

 shape when isolated than they do towards the close of their development. 

 G. Kraus erroneously concluded from this that the longitudinal strains 

 were greatest when growth was most active, whereas this might result 

 from the thickening or increased elasticity of the cell-walls without growth 

 decreasing at all. There is, however, a certain diminution of the strains 

 in most adult organs, although a resumption of growth or its acceleration 

 may cause them to increase again '. 



Strains in the cuticle and cell-wall. That the cuticle is under strain is shown 

 by the fact that the outward curvature of epidermal strips from Agave and 

 Hyarinthus is produced in water, even when only the outer wall is present. The 

 same takes place when a transverse section from an internode of Nitella or of a 

 pollen-grain is cut in two 2 . Again, the dissimilar character and power of swelling 

 of the different lamellae of the cell-wall is shown by the twisting and curving 

 occurring in the isolated outer wall of the epidermis, in moss peristomes, in 



1 On the tensions during peren nation in winter cf. G. Kraus, Bot. Zeitung, 1867, p. 118. 



2 Hofmeister, Jahrb. f. wiss. Bot., 1863, Bd. in, p. 82; Pflanzenzelle, 1867, p. 267. 



PFEFFER. II F 



