358 Morpho genetic Factors 



tied in a vertical loop, reaction wood is formed on the under side of both 

 the upper and lower parts of the loop. In the former, the wood is under 

 compression but in the latter under tension. That gravity is not directly 

 responsible is shown by the appearance of reaction wood on the upper 

 sides of branches which are being pushed down. In every case the de- 

 velopment of this wood is such that it will bring back the normal pattern. 



The reaction of each part of the plant seems to be a specific orienta- 

 tion to the direction of gravitational pull. This reaction is different in 

 different parts of the growth pattern of the tree. In herbaceous material 

 (branches of Aster) the author has found that a lateral branch tied out 

 of position will tend to assume its normal angle to gravity rather than 

 its normal angle to the main axis of the plant. When gravity is replaced 

 experimentally by centrifugal force, reaction wood is also produced 

 (Scott and Preston, 1955). 



Auxin has been shown to be responsible for the relation between a 

 terminal bud and lateral ones below it (p. 386), and it is presumably 

 concerned with the production of reaction wood (Wershing and Bailey, 

 1942). The problem of morphogenetic significance is why there is just 

 enough auxin (and thus enough reaction wood) at just the right place 

 and time to produce such a specific pattern of branching that this can 

 be used as a diagnostic character for the species. Here is the problem 

 of organic form in one of its simplest but most puzzling manifestations. 



Spurr and Hyvarinen ( 1954a ) have reviewed the literature of reaction 

 wood in the conifers. 



Tissue Tension. Another factor, mechanical in its nature, which may 

 be of some morphogenetic importance is tissue tension. Not all the cells 

 in a tissue are equally turgid, and cell walls differ in their elasticity and 

 their plasticity. Tissues also grow at different rates. These differences 

 often cause tensions between cells or groups of cells which, since plant 

 cells adhere firmly to each other, cannot be reduced by cellular readjust- 

 ments in position. 



In an early and thorough paper Gregor Kraus (1867) examined this 

 problem. He measured the length of a piece of growing herbaceous inter- 

 node and then sliced it into longitudinal strips, each consisting of only 

 one tissue (pith, wood, cortex, or epidermis). When he measured the 

 length of these, he found that the outer ones had shrunk in comparison 

 with their original length before isolation but that the inner ones had 

 expanded. Evidently there was considerable tension between them in 

 the intact internode, the outer tissues being stretched and the inner ones 

 compressed. 



The degree of tissue tension is not constant but is usually low in young 

 internodes, increases farther back, and finally decreases in most cases 

 to zero as growth finally ceases. The distribution of tension among the 



