IRRITABILITY. 455 



cases great care to distinguish true geotropic curvatures from 

 curvatures due to mere weight. It was mentioned in a 

 previous lecture (p. 342) that the pendent position of flower- 

 buds is frequently due to the pliability of the upper growing 

 portion of the peduncle. But this is not always the case, 

 and a definite opinion can only be formed in any particular 

 instance as the result of experiment. The decisive experiment 

 consists in causing the stalk bearing the pendent flower-bud 

 to rotate horizontally on the clinostat. If the curvature is due 

 to the weight of the bud, the bud will continue to hang down- 

 wards during rotation ; but if the curvature is geotropic then 

 the original curvature will be retained, so that the bud in the 

 course of each rotation will lie sometimes above, sometimes 

 below, and sometimes at the side of the straight portion of the 

 stalk. By experiments of this kind Vochting has ascertained, 

 for instance, that the pendent position of the buds of Galanthns 

 nivalis and of Helleborus is due to the weight of the bud being 

 too great for the stalks to bear erect ; the pendent position of 

 the buds of various species of Poppy and of Tussilago Farfara, 

 on the other hand, is due to geotropic curvature, a conclusion 

 which has been confirmed by Fiinfstiick as regards the Poppy. 



It will be convenient in dealing with the facts of geotropism 

 to take separately the phenomena presented by organs of dif- 

 ferent physiological properties. We will begin with those 

 presented by radial organs, and these we shall further subdivide 

 into those which are peculiar to orthotropic radial organs, and 

 those which are exhibited by plagiotropic radial organs. We 

 shall then consider those presented by bilateral organs, taking 

 first those exhibited by isobilateral organs, and then those 

 peculiar to dorsiventral organs. 



It is a familiar fact, with regard to orthotropic radial organs, 

 that primary shoots grow vertically upwards, and that primary 

 roots grow vertically downwards into the soil. Inasmuch as 

 this takes place at the most widely distant parts of the earth's 

 surface, the fact may be more precisely stated thus, that the 

 shoots grow outwards, that is, away from the centre of the 

 earth, and the roots inwards, that is towards the centre of the 

 earth. 



