Various Physical Factors 355 



and spongy parenchyma by reversing the orientation of Iris japonica 

 leaves to gravity. Kreh (1925) examined changes in the fruiting bodies 

 of the fungus Lenzites which had been turned through an angle of 180° 

 with relation to gravity and found that in new growth the original 

 dorsiventral structure was restored. Many other cases of the effect of 

 gravity on structure have been reported. 



It is often difficult to distinguish the effects of gravity from those of 

 light, since a change in the relation to one usually produces a change 

 in the relation to the other unless experimental conditions are carefully 

 controlled. Wiesner (1892c) in a later paper recognized that his earlier 

 results in anisophylly were due to light as well as to gravity, and 

 Bussmann ( 1939, 1941 ) found the same to be true of induced dorsiven- 

 trality in fern prothallia. 



Gravity presumably does not modify plant structure directly but acts 

 through its influence on other factors. It produces tension and compres- 

 sion of tissues with consequent effects on cell division and expansion. 

 The reaction of woody plants in developing stem tissues strong enough 

 to resist bending and wind sway has been mentioned, and there must be 

 the same regulatory control of growth to resist weight of trunk (Esser, 

 1946). In this sense, gravity acts as a mechanical factor. Opatowski 

 (1946) explains the oblique growth of trees under the action of prevailing 

 winds as a mechanical response, based on the concept of maximum 

 strength. 



Equally important is the role of gravity in the distribution of growth 

 substances. Just how this occurs is not clear, but differential distribution 

 of substances under the stimulus of gravity must be involved (p. 380; 

 Brauner, 1927). This presumably affects the form of structures when 

 symmetry is changed from radial to dorsiventral, for example. It also 

 has less direct effects. Van Overbeek and Cmzado ( 1948 ) and Fisher 

 (1957) have shown that flower formation is geotropically stimulated in 

 horizontally placed pineapple and soybean plants, presumably by 

 alteration in the distribution of growth substances. Other phenomena 

 of differentiation may perhaps be explained in the same way. 



Gravity serves as the frame of reference to which the whole pattern 

 of plant growth is regulated. A plant develops under the constant and uni- 

 form stimulus of gravity, and its tropistic and morphogenetic responses— 

 hardly to be distinguished from each other here— are such that a spe- 

 cific bodily form is produced. Without this regulation to a steady orient- 

 ing directive, the general pattern of the plant body would doubtless be 

 much less specific and might even fail to be developed. Because of their 

 being anchored in one spot, plants are much more sensitive of such 

 gravitational form control than are animals. 



Reaction Wood. The example of such control that has been worked 



