ACTION OF GRAVITATION ON GROWTH IN LENGTH. 
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 Growth in Length: — Geotropism^. 
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 obhque 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 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fest a different rapidity of growth if entirely withdrawn from the influence of gravi- 
tation (like positively heliotropic organs when withdrawn from the influence of light) 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 
^ Schmitz, 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 confirmed, in other cases, by 
Müller and F. Darwin (see afite),'] 
^ Knight, Phil. Trans. 1806, vol. I. pp. 99-108. — Johnson, Edinburgh Phil. Journ. 1828, p. 312. 
— Dutrochet, Ann. des Sei. Nat. 1833, p. 413. — Wigand, Bolan. Untersuch. Braunschweig 1854, 
p. 133. — Hofmeister, Jahrb. für wissensch. Bot. vol. III. p. 77. — Ditto, Bot. Zeitg. 1868, Nos. 16, 17, 
and 1869, Nos. 3-6. — Frank, Beiträge zur Pflanzen-Phys. Leipzig 1868, p. i. — Midler, Bot. Zeitg. 
1869 and 1871.- — Spescheneff, Bot. Zeitg. 1870, p. 65. — Ciesielski, Untersuch, über die Abwärts- 
krümmung der Würzein, Breslau 1871. — Sachs, Arbeit, des bot. Inst, in Würzburg 1872, Heft 2. 
Abh. 4 and 5. — Ditto, Exper.-Phys., p. 505. — Ditto, Flora, 1873, No. 21. [Darwin (Movements of 
Plants) terms whät 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, Helsingfors 1880) 
that when the sporangiophores oi Phy corny c es, 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 gi ow less 
rapidly in the direction of the action of gravity than in the opposite direction.] 
