664 REPORT— 1891. 



ments carried out in tlie WiirzLurg laboratory, tried to show that Frank's results 

 can be explained without having resort to new types of geo- or heliotropism. De 

 Vries believed, for instance, that a leaf may be apheliotropic and apogeotropic, 

 and that its horizontal position under vertical illumination is due to a balance 

 struck between the opposing tendencies, one of which calls forth an upward, the 

 other a downward curvature. 



The same point of view occurs again in Sachs' paper on ' Orthotrope and Pla- 

 giotrope Plant-members.'' Sachs holds to the opinion that Frank's theory is 

 untenable, that it is upset by De Vries, and that the oblique or horizontal position 

 is to be explained as the result of a balance between opposing tendencies. 



In a paper published the following year (1880) - 1 attempted to decide between 

 the opposing views. My experiments proved that at least certain leaves can place 

 themselves at right angles to the direction of incident light when there is no 

 possibility of a balance being struck. The outcome of my experiments was to 

 convince me that Frank's views are correct, namely, that the quality of growth 

 called transverse heliotropism does exist. 



This view was accepted by my father in the ' Power of Movement.' The 

 conclusions of Vochting, in the"' Bot. Zeitung,' 1888, and Krabbe in Pringsheim's 

 ' Jahrbiicher,' 1889, vol. xx. are on the same side of the question. 



The general result of these confirmations of Frank's conception has been to 

 bring to the front a belief in the individuality of the plant in deciding what shall 

 be the effect of external conditions. Such a view does not necessarily imply 

 irritability in a strict sense, for Frank himself explained the facts, as we shall see, 

 in a different way. But it could not fail to open our eyes to the fact that in 

 growth-curvatures as in other relations to environment external changes are 

 effective as guides or sign-posts, not as direct causes. 



Frank saw clearly that plants may gain such various aptitudes for reacting to 

 light and gravitation as best suit their modes of life. 



In stating this view he refers to the influence of the 'Origin of Species,' 

 which had shown how any qualities useful to living things might be developed by 

 natural .selection. Frank described the qualities thus gained under the term 

 polarity. He supposed that the cell-membranes of a transversely heliotropic leaf 

 (for instance) were so endowed that a ray of light striking it obliquely from 

 base to apex produced an increase of growth on the side away from the light : 

 while a ray oblique from apex to base caused a reverse movement. The polarity- 

 assumption of Frank is a purel}' gratuitous one, and, if strictly interpreted, hardly 

 tends to bring growth curvatures into harmony with what we know of the relation 

 of life to environment. 



It will no doubt appear to be a forcing of evidence if, after such a statement as 

 the last, I still claim for Frank that he led the way to our modern view of 

 irritability. I can of course only judge of the effect of his writings on myself, and 

 I feel sure that they prepared me to accept the modern views. It must also be 

 insisted that Frank, in spite of his assumption of polarity, seems to have looked 

 at the phenomena in a manner not very different to ours of the present day. 

 Thus he compares the action of gravitation on plants to the influence of the 

 perception of food on a chicken. He speaks too of custom,' or use, building up the 

 specialised ' instinct ' for certain curvatures. These are expressions consistent 

 with our present views, and I think that Vines* is perfectly just in speaking of 

 Frank's belief in different kinds of irritability, although in so judging he may 

 perhaps have followed equity rather than law. 



One of the chief bars to the development of our present views on irritability was 

 the fact that simple growth in length is influenced, and markedly influenced, by 

 differences in illumination. Plants grow more quickly, ccBteris paribus, in darkness 

 than in light. With this fact to go on, it was perfectly natural that simple 

 mechanical explanations of heliotropism should be made. De CandoUe, as ia 

 well known, explained such curvatures by the more rapid growth of the shaded 

 side. Thus it came about that heliotropism was discussed, for instance, in Sachs' 



' Sachs' Arheiten, 1879. = Journal Linn. Soc. 1880. 



' Loc. cit. p. 91. < Vines' Physiology. 



