354 Morpho genetic Factors 



stant wind and another in a windless environment. The former developed 

 50 per cent more volume of collenchymatous tissue than the latter. In all 

 such cases, however, it is difficult to separate mechanical effects from 

 those caused by increased water loss. 



Ultrasonics. The morphogenetic effect of a quite different mechanical 

 factor, intense ultrasonic vibration, has been reported by several work- 

 ers. Takashima and others ( 1951 ) found that in germinating radish seed- 

 lings exposed for 16 minutes the shoots were much deformed. In peas 

 similarly exposed, length of root and shoot was increased through in- 

 creased cell size. 



Gravity. Gravity is unlike the mechanical factors just discussed in 

 that it is continuous, unchanging in intensity, and constant in direction. 

 It is one of the most important formative factors, for plants must con- 

 tinually regulate their growth to it. The upright position of main-shoot 

 axes, the downward growth of primary roots, and the various inter- 

 mediate orientations of leaves and lateral branches and roots are mani- 

 festations of geotropic growth reactions. The general growth pattern of 

 the plant body is a specific reaction to gravity. The problem of tropisms 

 is primarily one for plant physiology but the student of morphogenesis 

 should not lose sight of the fact that these tropisms, whether reactions 

 to gravity, light, or other stimuli, are continually molding the pattern of 

 the plant. 



One can distinguish between the tropistic effects of gravity in the strict 

 sense, which involve the orientation of parts, and its truly formative ones. 

 Conspicuous among the latter are the modifications of symmetry from 

 radial to dorsiventral or vice versa (p. 176). Many years ago Wiesner 

 (1868) and others studied the dorsiventral symmetry of plant struc- 

 tures, especially leaves, when the axis on which they were borne was 

 horizontal instead of vertical, and succeeded in inducing form changes in 

 them experimentally. For such a difference in form between the upper 

 and the lower leaves of a horizontal branch or between the upper and 

 lower sides of such leaves, Wiesner proposed the term anisophyllij (p. 

 171). Goebel later distinguished between anisophyllous forms which are 

 constant and hereditary and those which can be reversed by changing 

 the relation of the growing structures to gravity. 



Gravity has been found to modify internal structure also. Brain (1939) 

 grew various plants horizontally on a clinostat and found that this modi- 

 fied the cells somewhat, those on the clinostat being generally shorter 

 and wider, presumably because of the greater extensibility of their cell 

 walls. Larsen (1953) found that gravity has little effect on rate of cell 

 elongation when acting on roots growing in the normal direction but 

 checks such elongation when acting in the opposite direction or at right 

 angles to it. Imamura ( 1931 ) was able to change the position of palisade 



