48] CHAPTER III. SPECIAL PHYSIOLOGY OF MOVEMENT. 217 



of Chara and Nitella, the peduncles of flowers, the stipes of the 

 larger Fungi, and the conidiophores of Moulds, as also radial 

 leaves such as those of the Onion, are positively heliotropic. 

 Negative heliotropism has been observed in many roots, especially 

 aerial roots, and in the root-hairs of Marchantia. With regard to 

 shoots, the hypocotyl of Viscum, the Mistletoe, is negatively helio- 

 tropic. 



Although the relation between the external symmetry of the 

 member and its heliotropic irritability is generally that indicated 

 above, yet there are exceptions : all dorsiventral members are dia- 

 heliotropic ; but not all radial members are positively or negatively 

 heliotropic, for some of them are diaheliotropic. It seems that 

 continual exposure to intense light falling on one side induces at 

 least physiological dorsiventrality in some radial members (e.g. 

 shoots of Ivy and Tropseolum). 



The flattened, typically isobilateral, leaves of various Monocoty- 

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



e. Irritability to the Directive Influence of Gravity (Geotro- 

 pism). 



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 primary 

 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, 

 the}- curve until it is so ; or if they be so moved that the normally 

 upper surface faces downwards, they twist until it faces upwards. 

 These members behave in respect to the line of action of gravity 



