248 TROPIC MOVEMENTS 



as the result of heliotropic stimulation. In the case of geotropically stimulated roots, 

 Kraus l found an increase in the percentage of water as usual on the convex side, 

 which is here the upper one, while in old non-curving roots geotropic induction pro- 

 duces the same rise in the percentage of water on the upper side, as is shown in the 

 lower side of non-curving old stems. 



Similar changes are very rapidly produced by shaking, for Kraus 2 found that 

 after shaking a growing defoliated shoot of Alliaria officinalis the amount of sugar 

 rose from 0-1463 to 0-1618 of a gram, while the side kept convex during shaking 

 contained sap of higher density and with a higher percentage of sugar. 



PART V 

 SECTION 55. Special Cases. 



Although the usual loss of the power of tropic curvature in adult 

 organs may involve a certain disadvantage, nevertheless it would need too 

 great an expenditure of energy and material to render the older parts of 

 a tree not only capable of supporting the other organs but also of per- 

 forming tropic movements. Hence the plant strives to adjust itself by 

 means of the new shoots, and allows the older organs to remain in positions 

 forced upon them. 



ROOTS. The primary geotropic curvature of the main root may 

 be more or less modified by hydrotropic, rheotropic, heliotropic, trau- 

 matropic, and aerotropic stimuli. In addition, obstacles may cause the 

 plastic apex to diverge temporarily from its attempted line of growth, 

 but the influence of all these factors upon the growth and shape of the 

 root-system does not require detailed discussion 3 . The avoidance of 

 obstacles does not appear to be the result of any contact stimulation but 

 may in extreme cases partly result from traumatropic excitation. Roots 

 can exercise a considerable downward pressure when lateral displacement 

 is prevented, and the pointed growing apex has a high power of lateral 

 expansion, as have also the older parts of the root during secondary 

 growth. The weight of the seed, or of a thin covering of soil, gives usually 

 a sufficient fulcrum for the downward pressure exercised by the radicle 

 in penetrating an ordinary soil. In many cases the formation of mucilage 

 or the early production of root-hairs aid in fixing the seed 4 , while an 

 increased leverage may be assured by the curvature assumed by the 

 hypocotyl. 



RHIZOMES are usually diageotropic, but certain forms may temporarily 



1 Kraus, Ueber die Wasservertheilung, &c., 1880, Bd. II, p. a6. 2 Kraus, 1. c., p. 69. 



8 Freidenfelt, Flora, 1902, Ergzbd., p. 115. 



4 Pfeffer, Druck- u. Arbeitsleistungen, 1893, pp. 362, 365, 369; and for the literature concerning 

 the escape of shoots from the soil, p. 383 ; also Areschoug, Beitr. z. Biol. d. geophilen Pflanzen, 

 1896. 



