LECTURE XL. 



THE ANISOTROPY OF THE ORGANS OF PLANTS'. 



I USE the above expression to denote the fact that the different organs of 

 the plant assume very various directions of growth under the influence of the same 

 external forces. It must be remembered in this connection that the direction in 



' The first attempt to describe and classify the vital phenomena of plants here considered, was 

 made by Hugo v. Mohl (1836) in his treatise ' Über die Symmetrie der Pflanzen,' contained in his 

 'Vermischten Schriften botanischen Inhalts'' (Tübingen. 1S45, P- ^2). Since that, scarcely any one 

 troubled himself further about these matters till I again brought the subject into notice in 1S70 in my 

 'Lehrbuch,' in the second and subsequent editions, in the paragraphs on the directions of growth. 

 Meanwhile Hofmeister had introduced great confusion into this province by neglecting all the facts 

 depending on the internal symmetry and other correlations of growth, and by ascribing the directions 

 of growth of the organs of plants to the influence of light and of gravitation, and especially in cases 

 where this had nothing to do with the matter ; but the worst was that Hofmeister, in the considera- 

 tion of the relations of direction of the lateral buds of woody plants, made a scarcely conceivable 

 error of observation in describing the buds which stand right and left as arising above and below 

 on the parent-axis, and drew conclusions therefrom. 



The description given in this lecture is based chiefly on my treatise, published in 1879, 'Über 

 orthotrope und plagiotrope /^^cwr^cw/Z/^z/^ ' (Arb. d. bot. Inst. Wzbg., B. II, 1882, p. 226), where I 

 first made clear the causal relations between orthotropic growth and radial structure, and between 

 plagiotropic growth and dorsiventral structure. 



More detailed statements, especially as to the directions of growth of the lateral shoots of the 

 Coniferje and Begonias are found in my ^Lehrbuch'' (Aufl. Ill and IV) in the paragraphs on direc- 

 tions of growth. 



Hydrotropism is treated of in my publication (1872), 'Ablenkung der Wurzel von ihrer 

 normalen Wach sthuinsrichtung durch feuchte Körper'' (Arb. d. bot. Inst. Wzbg., B. I, 1874); and in 

 my treatise ' tJber Ausschliessung der gcotropischen und heliotropischen Krihniimngen während des 

 ]Vacksthens' (Arb. d. bot. Inst. Wzgb., B. II, p. 209), I first pointed out that orthotropic shoots and 

 sporangiophores place themselves vertically with respect to the surfaces of a cube rotating on the klino- 

 stat, and that this is apparently a consequence of negative hydrotropism. This was then confirmed by 

 Wortmann in his treatise, 'Ein Beitrag zur Biologie der Mucorini'' (Bot. Zeitg., 1881, p. 368). 



The reader superficially acquainted with our literature may be somewhat surprised that I have 

 made no further mention in the preceding series of lectures of Darwin's book, 'The Power of 

 Movement in Plants' (London, 1880). I find myself with regard to this book in the most painful 

 position, and can only regret that the name of Charles Darwin appears on it. The experiments 

 which he, together with his son, describes, are made without sufficient practical knowledge, and are 

 badly interpreted, and what little good is found in the book concerning general views is not new. 

 The main conclusion to which Darwin comes in his book, that all the movements of irritability in 

 the vegetable kingdom depend upon 'circumnutation,' characterises better than anything else the 

 position which the two authors occupy. It is unnecessary to say anything on this subject. Detlefsen 

 has said sufficient as regards the brain-function of the growing-point of the root (Arb. d. bot. Inst. 

 Wzbg., B. II, 1 88 2, p. Ö27). 



