214 The Anatomy of Plants BOOKII 



gams and some other cases it has become collateral by 

 reduction. The pith is to be regarded as an enclosed 

 portion of the fundamental tissue. 



Jeffrey used the terms phyllosiphonic and cladosiphonic 

 to indicate the tubular steles with foliar and ramal gaps 

 respectively. 



The question of the secondary thickening of the stele 

 does not properly belong to the period under discussion, 

 but there are certain features which call for recognition. 

 The phenomena of the normal cambial activity in both 

 stem and root were known much earlier, and the con- 

 struction of the annual rings of wood had been observed 

 and studied by many writers before 1860. During the 

 years 1860-1900, however, many suggestions were made 

 with a view to explaining the different sizes of the 

 cells at the beginning and the closing of the annual 

 thickening period. In the Lehrbuch in 1868 Sachs assumed 

 that the smaller diameter of the autumn wood should be 

 attributed to the great increase of internal pressure brought 

 about by the continuous growth of the ring in so confined 

 a space. In 1872 De Vries artificially increased this pressure 

 by binding branches in the spring with ligatures of twine, 

 and found that in the succeeding period of growth the cells 

 underlying the ligatures were smaller than those beyond 

 them, --an experiment which was confirmatory of Sachs' 

 view. Ten years later Krabbe opposed the hypothesis, 

 saying that the pressure spoken of has no existence in the 

 normal condition of the tree. In 1880 Hartig, and later 

 in 1887 Wieler, attributed the change to the altered con- 

 ditions of nutrition that accompany the passing of the 

 summer. In 1881 Russow suggested that the greater dia- 

 meter of the spring wood-cells should be attributed to their 

 greater turgidity during development. Wieler, on the other 

 hand, found the turgidity equal in autumn and spring. 

 Haberlandt (1884), Strasburger (1891), and Hartig (1894) 



