SPECIAL CASES 249 



become positively geotropic, owing to an aitiogenic or autogenic change of 

 tone. The rhizomes of Adoxa and Circaea are, for instance, positively geo- 

 tropic when illuminated, but become diageotropic as soon as their downward 

 curvature below the surface of the soil brings them into darkness. It is, 

 however, not known whether differences in the distribution of oxygen, carbon 

 dioxide, water, and temperature may produce changes of tone or tropic 

 reactions regulating the depth of the rhizome, or whether the distance between 

 the rhizome and the subaerial parts influences the geotropic tone. The latter 

 is, in fact, strongly affected in certain rhizomes by the partial or complete 

 removal of the subaerial organs. Correlative reactions of this kind are 

 often of predominant importance, according to Rimbach *, although rhizomes 

 of the same plant under similar conditions may vary in depth within wide 

 limits. Miiller 2 , in fact, concludes that the depth of rhizomes is due solely 

 to extraneous circumstances such as the action of earth-worms and the 

 like. That such factors may influence the depth is certain, but it is hardly 

 possible in this way to explain all the phenomena observed 3 . An instance 

 of correlation is afforded by those cereals in which, according to Schellenberg, 

 the illumination of the leaves influences the development at the nodes to 

 which they are attached 4 . The development of contractile roots which 

 draw bulbs and corms deeper into the soil is possibly also the result of 

 correlative influences, as would also be the cessation of the formation of 

 these roots when an appropriate depth is reached. Naturally other factors 

 may also come into play in determining the position assumed, among 

 which the peculiar downward transference of the corms of young seedlings 

 of Crocus is included. 



AERIAL STEMS. Owing to the tonic and orienting actions of light, and 

 to the influence of such agencies as wind and moisture, the relationships 

 are here more complicated, while in addition the mere weight of the 

 organ may cause it to diverge more or less from the position which it 

 strives to assume. The erect position of the main axis is largely due 

 to its negative geotropism, while the lateral shoots either assume a plagio- 

 tropic position in virtue of their autotropism or are led into particular 

 positions by various aitiogenic influences. The latter applies more especially 

 to the leaves, and here the orienting action of light is naturally of primary 

 importance, although heliotropic stimuli may also influence stems to a 

 pronounced degree. 



RUNNERS AND CREEPING SHOOTS 5 . The horizontal or obliquely 



1 Rimbach, Beitrage z. wiss. Bot. von Fiinfstiick, 1899, Bd. ill, p. 177. 

 8 P. E. Miiller, Bot. Centralbl., 1896, Bd. LXVI, p. 22. 



3 Areschoug, Beitr. z. Biol. d. geophilen Pflanzen, 1896 ; Goebel, Organography, 1900, p. 224. 

 * Schellenberg, Unters. iiber d. Lage d. Bestockungsknoten beim Getreide, 1902, p. 21 (reprint 

 from Forschungen a. d. Gebiete d. Landw.). 



5 Frank, Die natiirl. wagerechte Richtung, etc., 1870, p. 17; Bot. Ztg., 1873, p. 36; Czapek, 



