CHAPTER III. SPECIAL PHYSIOLOGY OF MOVEMENT. 751 



under the influence of intense unilateral illumination, the radial organ becomes 

 physiologically dorsiveutral ; it is then diaheliotropic (not negatively helio- 

 tropic) in place of being positively heliotropic, and any curvature which it may 

 present is due to epinasty or hyponasty. 



The flattened, typically isobilateral, leaves of various Monocoty- 

 ledons, such as those of Iris, appear to be positively heliotropic. 



e. Irrit ability to the Directive Influence of Gravity (Geotropism). 



The effects of the stimulating directive action of gravity must 

 be clearly distinguished from those which are due to the mere 

 weight of the parts. It is only the former which are referred to 

 by the term geotropism. The geotropic curvatures are effected 

 with considerable force, and will take place even against consider- 

 able resistance ; for instance, it has been observed that the pri- 

 mary roots of seedlings will curve downward into mercury. 



Geotropic irritability is manifested by various members, such as 

 stems, leaves, and roots. The phenomena of geotropism in the 

 three categories of members, the dorsiventral, the radial, and the 

 isobilateral, will now be studied. 



With regard to dorsiventral members, it appears that many 

 leaves, both growing and motile, lateral shoots of Conifers and of 

 many dicotyledonous shrubs, runners, etc., which are dorsiventral, 

 take up such a position, when acted upon solely by gravity, that 

 their longitudinal axis is horizontal that is, at right angles to the 

 line of action of gravity, the vertical and that their morphologi- 

 cally superior surface is directed upwards. If these members are 

 moved out of this position so that their long axis is not horizontal, 

 they curve until it is so ; or if they be so moved that the nor- 

 mally upper surface faces downwards, they twist until it faces up- 

 wards. These members behave in respect to the line of action of 

 gravity just as they do to the direction of the incident rays of 

 light. They are diageotropic, just as they are diaheliotropic. 



It is a familiar fact that at all points of the earth's surface typi- 

 cal radial members, such as primary shoots and roots, grow with 

 their long axis vertical, but with this difference, that the direction 

 of growth of the primary shoots is away from the centre of the 

 earth, whereas that of the primary roots is towards the centre of 

 the earth. It can be readily demonstrated (by Knight's machine, 

 Fig. 483) that this vertical direction of growth is due to the force 

 of gravity, that it is, in fact, a phenomenon of geotropism. But 

 the effect produced is precisely opposite in the two cases ; primary 

 shoots grow in a direction opposed to that of the action of gravity, 



v. s. B. 3 c 



