1895.] Regulatory Formation of Mechanical Tissue. 443 
which Sachs and his school built their theory, we have the 
following eight statements, the truth of the last two of which is 
not without exceptions: 
1. Between all growing tissues both longitudinal and trans- 
verse tensions are present, the outermost tissues seeking to 
contract while the innermost are seeking to expand. 
2. Swaying of a plant promotes growth from the cambium. 
3. Ligatures about woody stems decrease the number and 
size of the xylem elements. 
4. Slitting the cortex longitudinally promotes growth from 
the cambium. 
5. Abrading the dead part of bark promotes growth from 
the cambium (Knight’). 
. Normal clefts in the bark of trees deepen in the early 
spring. 
7. Branches and roots of trees show excentricity of growth 
and the greatest growth is on the lower side, or when flexed 
on the concave side. 
8. In the curving of stems due to geotropism, heliotro- 
Hiem etc., the greatest growth in thickness is on the convex 
side, 
Granting for the moment that all of the foregoing state- 
ments are facts, it is rather surprising that the mechanical 
theory of growth could have gained such general credence 
when two of its vital supports were pure assumptions. In the 
first place it was necessary to assume that extension of cells 
is but the stretching of hydrostatic pressure from turgidity; 
and in the second place, that the resistance offered by the cor- 
tex is great enough to control the amount and direction of 
Cambium growth. 
In 1884 Krabbe published a research which showed the in- 
correctness of the second assumption, and last year Pfeffer 
showed the untenableness of the first. 
Krabbe® measured the compressing force of the cortex of 
many trees finding it to be always less than one atmosphere. 
Its increase of pressure from spring to autumn is never more 
than a fraction of one atmosphere while in many trees the in- 
Crease is hardly to be measured. : 
By using a ligature with graduated weight this botanist 
Knight 1. c. : 
*Krabbe, Ueber das Wachsthum des Verdickungsringes und der jungen Holz- 
zellen. Abhi. d. k. Akad. d. Wissensch. Berlin. 1884. 
