754 PART IV. THE PHYSIOLOGY OF PLANTS. 



long axis of the plant perpendicular to the plane of the wheel, each part is ex- 

 posed for successive equal times to the action of gravity in precisely opposite 

 directions, so that the directive action of gravity neutralises itself. The result 

 is that a stem or root with its long axis horizontal remains perfectly straight 

 when rotated on the clinostat, provided that other disturbing influences (light, 

 etc.) are excluded ; and the fact that the empirical results given by the machine 

 exactly correspond to what would be theoretically anticipated on the hypothesis 

 that gravity is the active directive force in geotropic curvature, is strong con- 

 firmatory evidence of the conclusions to be drawn from experiments with 

 Knight's machine. 



The geotropic influence of gravity is greatest when the radial 

 member is in a horizontal position; that is, the curvature into the 

 normal position then takes place with the greatest rapidity. But 

 the visible effect is the more marked, the further the member is 

 removed from its normal position ; for instance, when a primary 

 shoot is turned upside down, a curvature of 180 has to be per- 

 formed in order that the apex may again point upwards. 



In addition to the primary shoots of seedlings, the following radial members 

 are negatively geotropic ; the stipes of Mushrooms, the gonidiophores of Moulds, 

 the stems of Characese, the stalks of the receptacles of Liverworts, the peduncles 

 of many flowers, the setse of Mosses, etc. Also isobilateral leaves, such as those 

 of Iris ; when placed horizontally in darkness, whether flat or edgeways, they 

 curve upwards. 



In addition to the primary roots of seedlings, the following radial members 

 are positively geotropic ; the hyphae of Fungi which penetrate into the sub- 

 stratum, the root-like filaments of Vaucheria, and other Algse, the rhizoids of 

 Muscineae, the rhizomes of Yucca filamentosa and of Cordyline rubra, etc. 



An instance of the absence of geotropic irritability in a growing member is 

 afforded by the hypocotyl of the Mistletoe. 



The degree of geotropic irritability is not the same in all radial 

 members. It may be generally stated that the lateral branches 

 both of shoots and roots are less irritable than primary shoots and 

 roots. For instance, the secondary branches of roots grow, not 

 vertically downwards, but obliquely outwards and downwards, in 

 the soil. 



All radial members are not, however, either positively or nega- 

 tively geotropic ; some, such as the rhizomes of Heliocharis palustris, 

 Sparganium ramosum, and Scirpus maritimus, are diageotropic. 



It has been observed in some cases that the nature of the geo- 

 tropic irritability of a member may change in the course of its 

 development. For instance, the peduncle of the Poppy is posi- 

 tively geotropic whilst the flower is in the bud, but negatively 

 geotropic during flowering and fruiting. Again, the flowers of 



