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LECTURE XVIII. 



far as to make this generalisation, that shoot-organs are 

 negatively, and root-organs are positively somatotropic. 



But the observations before us, valuable as they are, are not 

 quite conclusive, for the directive influence of gravity was not 

 eliminated. In the case of the Mistletoe this is not a point of 

 great importance, for, as has been already mentioned (p. 458), 

 the hypocotyl is scarcely at all geotropic, but in the case 

 of the Fungi it is of importance for their hyphae are markedly 

 geotropic. It is easy to imagine that although an organ may 

 tend to conform to this law of perpendicular growth, yet its 



FIG. 52 (after Sachs). A, the axle of rotation ; T, the cube of turf; a k, seed- 

 lings of Lepidium sativum and Linuni iisitatissimum developed during 

 rotation ; m m 3 , sporangiferous hyphae of Mucor Mucedo. The obliquity of 

 the organs growing on the flanks of the cube is due to the throwing of a 

 shadow by the thick axle. 



irritability to the action of other directive influences may be 

 such as to induce effects which entirely mask the effort towards 

 perpendicular growth. The influence of light can be eliminated, 

 either by keeping the plant in darkness, or by causing the 

 plant to rotate slowly round either a vertical or a horizontal 



