75 y MECHANICAL LAWS OF GROWTH. 



Sect. 21. — Influence of Gravitation on Growth: — Geotropism^ It has 



already been shown in Sect. 10 that, when the access of Hght is equal on all sides or 

 when heliotropisra 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 direc- 

 tion 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 geoiropic, those which are 

 accelerated negatively geotropic 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 fest a different rapidity of growth if entirely withdrawn from the influence of gravita- 

 tion (like positively heliotropic organs when withdrawn from the influence of light) 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 affects the rapidity of growth when its 

 direction cuts that of the axis of growth at an angle, and the more so the nearer 

 the angle approaches a right angle. 



The positive or negative character of geotropism depends as little as that of 

 heliotropism on the morphological nature of the organ. Not only, for example, are 

 all the primary roots of the seedlings of Phanerogams positively geotropic, and most 

 secondary roots which spring from underground stems, as tubers, bulbs, or rhizomes ; 

 but also many leafy lateral shoots, especially those which are destined to produce 

 rhizomes or to form new bulbs {e.g. Tulipa, Physalis, Polygonum, &c.), and even 

 fohar structures, like the cotyledonary sheaths of Allium, Phoenix, and many other 

 Monocotyledons. Among positively geotropic organs must also be included the 

 lamellae and tubes of the hymenium of Hymenomycetous Fungi. All axes which grow 

 upright (and are not bilateral), petioles, and the stipites of many Hymenomycetous 

 Fungi, exhibit, on the other hand, decidedly negative geotropism. 



The geotropism, like the heliotropism, of different organs varies in all degrees. 

 It is, for example, manifested very strongly in the primary roots and upright 



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

 the phenomena of heliotropism. 



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

 — Dutrochet, Ann. des Sci. Nat. 1S33, p. 413. — Wigand, Botan. Untersuch. Braunschweig 1854, 

 p. 133. — Hofmeister, Jahrb. fiir wissensch. Bot. vol. Ill, p. 77. — Ditto, Bot. Zeitg. 1868, Nos. 16, 17, 

 and 1869, Nos. 3-6. — Frank, Beitriige zur Pflanzen-Phys. Leipzig 1868, p. 1. — Midler, Bot. Zeitg. 

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

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

 Abh. 4 and 5. — Ditto, Exper.-Phys., p. 505. — Ditto, Flora, 1873, No. 21. 



