TBANSACTIONS OF SECTION D. 663 



geotropism is an active, not a passive, curvature, and that it depends, like apo- 

 geotropism, on unequal distribution of longitudinal growth. Here, ao-ain, he 

 introduced unity, bringing what had been considered different phenomena under 

 a common heading. By studying the distribution of growth and of tension in a 

 variety of curvatures he helped still more to unite them under a common point 

 of view. 



(ii.) He showed that Ilofmeister's classification of organs into those (I) which 

 have and (2) which have not tension, was valueless in connection with growth- 

 curvatures ; that is to say that apogeotropism is not necessarily connected with 

 the form of longitudinal tension found in growing shoots, and that the distinct kind 

 of tension existing in roots has no connection with their positive geotropism. His 

 work thus served to bring the subject into a more purely physiological condition,' 

 not only by his downright opposition to a mechanical tlieory backed by the great 

 name of Hofmeister, but also by giving importance to physiological individuality. 



In 1870 Frank published a more important work, ' Die natiirliche wagerechte 

 Eichtung der Pflanzentheilen.' This paper not only tended to unite geotropism 

 and heliotropism by proving the phenomena described to be common to both 

 categories, but it more especially widened the field of view by showing that 

 horizontal growth must be considered as kindred to vertical growth, and thus 

 introduced a new conception of the reaction of plants to light and gravitation 

 which has been most fruitful. 



Frank showed that certain parts of plants, for instance the runners of the 

 strawberries, even when kept in the dark, grow horizontally, and when displaced 

 from the horizontal returned to it. Here, said Frank, is a new type of geotropism, 

 neither positive nor negative, but trans verse. Ten years later Elfving,' working in 

 Sachs' laboratory, got similar results with rhizomes of Scirpus, &c. These experi- 

 ments are more conclusive than Frank's in one way, because the strawberry runners 

 when darkened were in abnormal conditions, whereas the rhizomes used by Elfvino- 

 were normally freed from light-effects. When a rhizome which has been placed 

 so as to point obliquely upwards moves down towards the horizontal position it 

 is, according to the old nomenclature, positively geotropic, and, vice versa, when it 

 reaches the horizontal from below it is negatively geotropic. But it cannot be both 

 positively and negatively geotropic. We are bound to assume that it is so organised 

 that it can only assume a position of rest, and continue to grow in a straight line 

 when it is horizontal, just as an ordinary geotropic organ cannot devote itself to 

 rectilinear growth unless it is vertical, 'in this way Frank's conception of trans- 

 verse geotropism paved the way for the theory that there are a variety of different 

 organisations (or, as we may now say, irritabilities) in growing plants ; and that, 

 whether a plant grows vertically upwards or downwards or horizontally, depends 

 on the individual and highly sensitive constitution of the plant in question. It is, 

 of course, true that those who seek for mechanical explanations of growth curva- 

 tures might be able to find such a one for transverse geotropism. But when Frank's 

 conception has once been seized such views are less and less acceptable ; and, judg- 

 ing from my own experience, I cannot doubt that Frank's work deserved to have 

 a powerful effect in preparing the minds of physiologists for a just view of 

 irritability. 



The belief in transverse geotropism received interesting support from Vochting's' 

 work on the movement of certain flowers which retain a horizontal position imder 

 the influence of gravitation. 



Frank's views, it may be added, were accepted by my father and myself in our 

 * Power of Movement,' in which the term diageotropism was proposed, and has 

 been generally accepted, for transverse geotropism. Nevertheless, though Frank 

 was undoubtedly right, his views were strongly opposed at the time. He held 

 similar views on the effect of light, believing that the power possessed by leaves of 

 placing themselves at right angles to the direction of incident light must be con- 

 sidered as a new type of heliotropic movement, transverse or diaheliotropism. 

 Frank's views were criticised and opposed by De Vries,^ who, by means of experi- 



' Elfving, Sachs' Arheiien, 1880. ^ Die Bemegung der BUithen vnd Frilchte, 1882. 

 ' De Vries, Sachs' Arheiten, 1872. 



