4/0 . LECTURE XVIII. 



orthotropic radial shoots. Radical leaves, and the shoots of 

 Marchantia, afford us examples of permanently dorsiventral 

 organs which are negatively geotropic. 



But all organs which are dorsiventral, and therefore plagio- 

 tropic under normal conditions of growth, do not behave in 

 this way. Frank has observed, for instance, that the runners 

 of Fragaria lucida, the lateral branches of Conifers and of many 

 dicotyledonous shrubs and trees maintain their plagiotropic 

 habit even in darkness. When they are placed with their apices 

 directed upwards or downwards, they curve so as to assume a 

 more or less horizontal direction. And further, when they 

 are placed horizontally in the inverse position so that their 

 morphologically inferior surfaces are directed upwards and 

 their morphologically superior surfaces downwards, they twist 

 on their own axes until the normal relation of these surfaces 

 with respect to the vertical is attained. Many leaves were 

 also found to behave in this way. 



In considering the geotropic phenomena presented by 

 these dorsiventral organs, two facts are to be clearly dis- 

 tinguished from each other, namely, the maintenance of the 

 horizontal direction of growth, and the maintenance of the 

 normal relative position of the two opposed sides of the organ. 

 As far as the former is concerned these organs behave pre- 

 cisely like those radial organs which we have already con- 

 sidered (p. 463) : the latter is a peculiarity of dorsiventral 

 organs. The maintenance of the horizontal position in both 

 cases may be ascribed to Frank's diageotropism: but there 

 is this difference between the diageotropism of plagiotropic 

 radial organs and that of dorsiventral organs, that, in the 

 former case, it is indifferent which side of the organ lies upper- 

 most, whereas in the latter there appears to be a tendency to 

 maintain the morphologically superior surface uppermost in 

 all cases, and the morphologically inferior surface undermost. 



The assumption of the existence of diageotropism has 

 naturally been exposed to a good deal of criticism ; but this 

 criticism has, so far, been confined to the diageotropism of 

 dorsiventral organs. With regard, first of all, to the main- 

 tenance of the horizontal direction of growth, de Vries has 



