442 The Botanical Gazette. [October, 
being caused by the flow thitherward of nutritive fluid, made 
possible by the reduction of tension on the convex side. 
It will readily be seen that while Knight assumed tension 
of tissues to be a controlling factor in the growth in diameter 
of trees, Kraus extended this causal relation to all kinds of 
increase of tissue. It remained now for that master-builder, 
Sachs, in the first edition of his Lehrbuch, in 1868, 
out of the material furnished by his pupil Kraus, by Hof- 
meister, and by Knight, to construct the framework of a 
mechanical theory of growth, which he subsequently elabo- 
rated in his later manuals, after the researches of de Vries 
and Detlefsen. ‘ 
De Vries, * in the period from 1872 to 1876, had experi- 
mented in increasing and decreasing the pressure of the 
cortex of trees by winding about stems ligatures of twine in 
the first case, and by making longitudinal slits in the cortex 
in the second case. With his ligatures he obtained, so he 
believed, autumn xylem in the spring time; and by slitting 
he claimed to produce spring xylem in the autumn. His con- 
clusion is obvious: cortical pressure must be the cause of 
the formation of annual rings. : 
Detlefsen,® another pupil of Sachs, published in 1881 his 
observatioris and deductions on excentric growth, and starting 
with the premises that all expansion of cells is due to the 
hydrostatic pressure of the cell-contents and that any external 
resistance to this internal pressure would diminish the effect- 
iveness of the hydrostatic pressure, he drew the conclusion 
all excentricities of growth in stems and roots of woody plants 
according to the increase or decrease of the cortical pressure. 
sion reduced, but a longitudinally convex increased. 
Bringing together in smal] space the chief observations on 
5De Vries, Flora —? 241. 1872; —: 97. 1875: —: 2. 1876.—De l'influence de 
la pression du liber sur la structure des couches ligneuses annuelles. Extrait 
d. Archives Neerlandaises i: —. 1876, 
*Detlefsen, Versuch einer mechanischen Erklarung des excentrischen Dick- 
enwachsihum verholzter Achsen und Wurzeln. Arbeit. d. bot. Inst. in Wirz- 
burg 2: 670-87. de 
