48] CHAPTER III. SPECIAL PHYSIOLOGY OF MOVEMENT. 219 



of the natural phenomena is also a purely physical force, and the force 

 of gravity is the one which meets all the necessary conditions. 



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 conidio- 

 phores 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 mem- 

 bers are positively geotropic ; the hyphse of Fungi which penetrate into 

 the substratum, the root-like filaments of Vaucheria and other Algse, the 

 rhizoids of Muscinese, 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. 



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 

 the Daffodil are negatively geotropic when in the bud, but they 

 become diageotropic as they open. 



/. Irritability to Differences in the degree of Moisture in the 

 surrounding Medium (Hydrotropisrn}. 



Irritability of this kind is especially characteristic of earth-roots 

 which possess it in a high degree. It can be readily demonstrated 

 by a well-known experiment. Peas or Beans are made to ger- 

 minate in a sieve full of damp sawdust, the sieve being suspended 



