156 TROPIC MOVEMENTS 



Sachs l used the terms parallelotropic (orthotropic) and plagiotropic more 

 to distinguish between perpendicular and horizontal organs, but they may be 

 used in a general sense to indicate the mode of orientation to any directive 

 agency, and if necessary the latter can be indicated by the usual prefixes as 

 in the terms geoparallelotropic, photoplagiotropic, aitiotropic and autotropic. 

 The words orthotropic and campylotropic or skoliotropic have been used 

 to indicate whether an organ is straight or curved -, and hence Sachs' use 

 of the term orthotropic seems inadvisable. 



The natural positions of the different organs are not solely due to 

 tropic stimuli, and in fact many organs have no tropic irritability, while in 

 all cases the autotropic tendencies of the organs come more or less into 

 play. Tropic irritability is naturally most strongly developed in the 

 organs where it is of greatest importance, and may be mainly or solely 

 responsible for the orientation of various parts. Since the different tropic 

 irritabilities may occur singly as well as in combination, it is evident that 

 each involves a definite form of sense-perception. Hence one positively 

 geotropic organ may be also positively heliotropic, but another may show 

 negative or plagio-heliotropism, while yet another may be devoid of one form 

 of irritability, or may have it modified without affecting its other senses 3 . 



Even in non-cellular plants the different organs develop varying 

 irritabilities, and the strong heliotropic irritability of the sporangiophore of 

 Phycomyces is absent from the hyphae. Changes of tone of internal or 

 external origin may also modify the result obtained by stimulating reacting 

 organs, as when the absence of light causes a dia-geotropic organ to 

 assume a klinotropic or parallelotropic position. In addition a rise in the 

 intensity of the stimulus may alter the orientation, as when a sufficient 

 increase of illumination causes the positively parallelotropic position of the 

 filaments of VancJieria, the sporangiophore of Phycomyces, and the young 

 shoot of various flowering plants to be replaced by a plagio-heliotropic one. 

 All plants do not show such pronounced reactions, but nevertheless in all 

 cases the existent and pre-existent conditions have a considerable influence 

 upon the irritable tone. 



Many radial organs may react plagiotropically, for the filaments of 

 Vaucheria and Phycomyces, lateral roots of the first order, as well as the 

 rhizomes of Heleocharis, Sparganium, Scirpus, and Agropyrnm, and the 

 runners of Lysimachia numtnularia, Glechoma and Vinca are not only 



any need for a special term in preference to the general one of ' tropism.' In the case of an organ 

 which partly twists and partly curves towards the light it might become necessary to say that it 

 possessed a positively paralleloheliotropocampylostrophismic (tortismic) irritability.] 



1 Sachs, Arh. d. bot. Inst. in Wiirzburg, 1879, Bci - n > P- 22 7- 



2 Czapek, Jahrb. f. wiss. Bot, 1895, Bd. XXVII, p. 312. 



3 A few instances in regard to geotropism and heliotropism are given by Frank, Beitrage zur 

 Pflanzenphysiologie, 1868, p. 89. 



