LEAVES WITH REGABD TO LIGHT. 421 



besides the ordinary and well known forms of growth, geotropism 

 and heliotropism, through which organs place themselves in the 

 line of the earth's radius, or parallel to the incident light, as the 

 case may be, there are special forms of growth, called transverse 

 geotropism and transverse heliotropism, An organ which grows 

 transverse-geotropically places itself horizontally ; it has an 

 inherent tendency to be horizontal, in the same way that the hypo- 

 cotyl of a seedling has an inherent tendency to grow vertically 

 upwards. In the same way a transversely heliotropic organ has 

 an inherent tendency to place itself at right angles to, instead of 

 parallel to, the direction of incident light. It may be said that 

 this is no explanation at all ; and this is true in a certain sense. 

 But there is no reason why it should be more unsatisfactory than 

 the accepted explanation of the vertical growth of stems and 

 roots, namely that they have an inherent power of growing in 

 these directions *. Another explanation of the facts is that given 

 by De Vries, in his paper " On the Direction of Bilaterally 

 Symmetrical Organs" f. The work in question is in great measure 

 a criticism on Frank's views, and proposes a theory of an entirely 

 opposite character. De Vries Believes it to be quite unnecessary 

 to assume the existence of special forms of growth as Prank 

 has done, and that the old forms of growth, positive and nega- 

 tive geo- and heliotropism, can, under certain conditions, account 

 for oblique or horizontal growth. He thinks that Frank has 

 neglected a possible explanation of the phenomena in the com- 

 binations and antagonisms which occur among the ordinary 

 heliotropic and geotropic forces. Thus an organ might be 

 apheliotropic J and apogeotropic, and when exposed to a zenith- 

 illumination might remain horizontal in consequence of the 

 balance between the apheliotropic tendency to grow away from 

 the zenith, and the equal and opposite apogeotropic tendency to 

 grow towards it. 



De Vries' s work also shows the existence of two forms of 

 growth, independent of gravitation and light, which, he believes, 



* The essential part of Frank's doctrine is accepted by my father in ' The 

 Power of Movement in Plants,' p. 438. 



+ 'Ueber einige Ursachen der Riehtung bilateralsynunetriseher Pflanzen- 

 theile,' Arbeiten des bot. Instituts in Wurzburg, Heft ii., 1872. 



I Apheliotropism is a convenient term, used instead of negative heliotropism. 

 When used without the prefix apo-, positive heliotropism is meant. Sec ' The 

 Power of Movement in Plants,' loc. cit. 



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