ACTION OF GRAVITATION ON GROWTH IN LENGTH. 839 



It is evident that we are here confronted with an unsolved problem ; and when all 

 the facts have been taken into consideration, the theory that there are two kinds of 

 cells, the growth of one of which (positively heliotropic) is retarded by light, whilst 

 that of the other kind (negatively heliotropic) is promoted by it, may be the simplest 

 and most in accordance with facts. This difference is the less remarkable since in the 

 behaviour of growing cells with respect to gravitation we find a precisely similar differ- 

 ence, but much more strongly marked ^ 



Sect. 21. — Action of Gravitation on G-rowth in Length: — G-eotropism^. 



It has already been shown in Sect. 10 that, when the access of light is equal on all 

 sides or when heliotropism is prevented by the exclusion of light, gravitation is the 

 cause of certain organs turning downwards, others upwards, and others again in 

 a direction oblique to the horizon. At present we shall speak only of those which 

 take a direction directly upwards or downwards, since other causes co-operate to 

 bring about an oblique growth. 



Just as organs, according to their internal nature, grow either more rapidly or 

 less rapidly on the side which faces the source of light than on the other side, so also 

 gravitation effects, in accordance with the nature of the organs, either an acceleration 

 or a retardation of growth on the side which faces the earth. Those organs which 

 are thus retarded in their growth are called positively geolropic^ those which are 

 accelerated negatively geoiropic organs. Positively geotropic organs consequently 

 become concave on the under side, and direct their growing apex downwards if 

 their axis of growth is brought into a horizontal or oblique direction ; negatively 

 geotropic organs, on the contrary, become convex on the under side under similar 

 conditions, and elevate their growing apex until it stands erect. 



It has not yet been ascertained whether positively geotropic organs would mani- 

 fest a different rapidity of growth if entirely withdrawn from the influence of gravi- 

 tation (like positively heliotropic organs when withdrawn from the influence of light) 

 from that displayed when gravitation acts in a direction parallel to the axis of 

 growth^. It would seem however as if gravitation only affected the rapidity of 



^ SchmitK, Linnsea, 1843, p. 513 et seq. If, as can scarcely be doubted, Schmitz's statements 

 with regard to Rhizomorphs are confirmed, it results that no certain inference can be drawn as to 

 the positive heliotropism of an organ from the fact that its growth is more rapid in the dark. We 

 could scarcely have a better proof of the necessity for a fresh and more accurate investigation of all 

 the phenomena of heliotropism. [Schmitz's observations have been confinned, in other cases, by 

 Miiller and F. Darwin (see ante).'] 



^ Knight, Phil. Trans. 1806, vol. I. pp. 99-108.— Johnson, Edinburgh Phil. Journ. 1828, p. 312. 

 — Dutrochet, Ann. des Sci. Nat. 1833, p. 413. — Wigand, Botan. Untersuch. Braunschweig 1854, 

 p. 133.— Hofmeister, Jahrb. fiir wissensch. Bot. vol. III. p. 77.— Ditto, Bot. Zeitg. i8f)8, Nos. 16, 17, 

 and 1869, Nos, 3-6.— Frank, Beitrage zur Pflanzen-Phys. Leipzig 1868, p. i.— MuUer, Bot. Zeitg. 

 1869 and 1871.— Spescheneff, Bot. Zeitg. 1870, p. 65.— Ciesielski, Untersuch. iiber die Abwarts- 

 kriimmung der Wiirzeln, Breslau 1871. — Sachs, Arbeit, des bot. Inst, in Wurzburg 1872,. Heft 2. 

 Abh. 4 and 5.— Ditto, Exper.-Phys., p. 505.— Ditto, Flora, 1873, No. 21. [Darwin (Movements of 

 Plants) terms what are here termed positive and negative geotropism, ' geotropism ' and • apogeo- 

 tropism ' respectively. He considers that both positive and negative geotropism are modified forms 

 of circumnutation {see infra),'] 



^ [Elfving has found (Beit.z. Kennt. d. physiol. Einwirkung der Schwerkraft, itelsingfors 1880) 

 that when the sporangiophores oi Phy corny ces, which are negatively geotropic, are grown in an inverted 

 ^position, their growth is not so rapid as it is under ordinary conditions ; that is, that they grow less 

 rapidly in the direction of the action of gravity than in the opposite direction.] 



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