366 PHYSIOLOGY OF GROWTH. 



These and other observations always showed, what is not in com- 

 plete harmony with the results obtained by H. de Vries, that the 

 zone of greatest extensibility lies in the youngest region of the 

 structure investigated, and is not coincident with the zone of most 

 vigorous growth. In the youngest zones, however, as Wortmann 

 determined (Bot. Zeit., 1889, p. 250), the osmotic pressure is 

 relatively small ; and hence, in spite of the great extensibility of 

 their membranes, they still do not grow very energetically. The 

 maximum of growth falls in a zone whose cells are still very 

 extensible, and develop high osmotic pressure (hence here also 

 the turgor- extension of the cells is greatest) ; while in the older 

 zones the growth is again slower, notwithstanding the fact that 

 the osmotic pressure of the cells keeps high, because the extensi- 

 bility of the membranes falls off very considerably. 



1 The fundamental ideas of our current theory of growth have been devel- 

 oped by Sachs. See my Lehrbuch der Pjianzenphysiologie, p. 213. Respecting 

 the experiments indicated, the student is referred to the work of H. de Vries, 

 Ueber mechanische Ursachen der Zellstreckung, Halle, 1877. See also Wort- 

 mann, Botan. Zeitung, 1889. 



148. The Contraction of Roots. 



In many plants careful observation shows that while the seed- 

 lings expand their cotyledons above ground, and the plumule 

 projects more or less out of the soil, the points of insertion of the 

 cotyledons and of the leaves developed from the bud are at a later 

 stage concealed in the soil. This subsequent dragging into the 

 soil of the points of insertion of the leaf structures can only be 

 caused by contraction of the root ; and, in fact, the occurrence of 

 such a phenomenon has been definitely determined by H. de Vries. 1 

 The contraction, whose biological significance is to be sought in 

 the protection which it affords to the buds in the soil, is due 

 to peculiarities in the growth of the roots. In the cells of the 

 parenchymatous tissue of the roots occurs energetic turgor. This 

 must be the case before growth can take place at all. But since 

 in the somewhat older roots the extensibility of the membranes 

 of the cells is greater in the transverse direction than in a 

 direction parallel to the long axis of the roots, the extension 

 of the cells due to turgor is actually greater in the former 

 direction than in the latter, and contraction of the organs must 



