August 27, 1891] 



NATURE 



409 



botanical mind should have taken more than fifiy years to 

 assimilate Dutrochet's view. 



In 1868 Albert Bemhard Frank published his valuable 

 " Beitrage zur Pflanzenphysiologie," which was of importance 

 in more than one way. In this work the term "geotropism" 

 was first suggested in imitation of the existing expression 

 " hcliofropism." This uniformity of nomenclature had an 

 advantage beyond mere convenience, for it served to emphasize 

 the view that the curvatures were allied in character. His criti- 

 cisms of Hofmeister and Sachs were directed against the follow- 

 ing views : — 



(i.) That roots and other positively geoiropic organs 

 bend owing to plasticity. By repeating and varying certain 

 older experiments, Frank helped materially to establish the now 

 universally accepted view that positive geotropism is an active, 

 not a passive, curvature, and that it depends, like apogeotropism, 

 on unequal distribution of longitudinal growth. Here, again, he 

 introduced unity, bringing what had been considered difterent 

 phenomena under a common heading. By studying the dis- 

 tribution 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 Hofmeister's classification of organs into 

 those (l) which have and (2) which have not tension, was value- 

 less 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 dis- 

 tinct 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 theory backed by the 

 i;reat name of Hofmeister, but also by giving importance to 

 physiological individuality. 



In 1870, Frank published a more important work, " Die natiir- 

 liche wagerechte Richtung der Ptlanzentheilen." This paper 

 not only tended to unite geotropism and heliotropism by proving 

 the phtnomena 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 re- 

 turned to it. Here, said Frank, is a new type of geotropism, 

 neither positive nor negative, but transverse. Ten years later 

 Elfving (Sachs's Arbeiten, 1880), working in Sachs's 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 Elfving were normally freed from 

 light-effects. When a rhizome which has been placed so a'^ to 

 point obliquely upwards, moves down towards the horizontal 

 position it is, according to the old nomenclature, positively ! 

 geotropic, and, -vice virsa, when it reaches the horizontal from 

 below it is negatively geotropic. But it cannot be both posi- 

 tively and negatively geotropic. We are bound to assume that it 

 is so organized 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 

 transverse geotropism paved the way for the theory that there 

 are a variety of different organizations (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 curvatures 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, judging from my own experience, I cannot 

 doubt that Frank's work deserved to have a powerful effect in pre- 

 paring the minds of physiologists for a just view of irritability. 



The belief in transver.-e geotropism received interesting sup- 

 port froin Vochting's work ("Die Bewegung der Bliithen und 

 Frvichte," 1882) on the movement of certain flowers which 

 retain a horizontal position under 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 



NO. II 39, VOL. 44] 



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 considered as a new 

 type of heliotropic movement, transverse or diaheliotropism. 

 Frank's views were criticized and opposed by De Vrifs (Sachs's 

 Arbeiten^ 1872), who, by means of experiments carried out in 

 the WUrzburg Laboratory, tried to show that Frank's results 

 can be explamed 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 posi- 

 tion under vertical illumination is due to a balance struck between 

 the opposing tendencies, one of which calls forth an upwani, the 

 other a downward curvature. 



The same point of view occurs again in Sachs's paper on 

 "Orthotrope and Plagiotrope Plant-members" (Sachs's^rMV^w, 

 1879). Sachs holds to the opinion that Frank's theory is un- 

 tenable, 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 (Journal Linn. 

 Soc), I 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 Move- 

 ment." The conclusions of Vdchting, in the Bot. Zeiturig, i888, 

 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 concep- 

 tion 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 ob- 

 liquely 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 purely g'atuitous 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. 1 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 msisted that Frank, inspiteof his assumption of polarity, 

 seems to have looked at the phenomena in a manner not very 

 different from ours of the present day. Thus, he compare-; the 

 action of gravitation on plants to the influence of the perception 

 of food on a chicken. He speaks, too, of custom (Journal Linn. 

 Soc, 1880, p. 91), or use, building up the specialized "instinct " 

 for certain curvatures. These are expressions consistent with 

 our present views, and I think that Vines ("Physiology") 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 in- 

 fluenced, and markedly influenced, by differences in illumina- 

 tion. Plants grow more quickly, eatais 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 Candolle, as is well known, explained such curva- 

 tures by the more rapid growth of the shaded side. Thus it 



