INFLUENCE OF CHEMICAL AGENCIES ON GROWTH in 



reawakened in the nodes of adult grass-haulms l , in which therefore gravity 

 seems to induce a premature cessation of growth when it acts parallel to 

 the long axis of the stem. 



As far as can be judged, gravity acts mainly as an orienting stimulus, 

 which is not generally essential. Hence plants are able to grow when its 

 orienting action is eliminated, as in free-swimming algae, bacteria, and 

 zoospores 2 . Similarly the mistletoe frequently grows in all directions in space 

 from the boughs on which it is fixed, and those runners which grow erect 

 or horizontally according to the intensity of the illumination, are only 

 feebly or not at all influenced by gravity. Indeed many of the higher 

 plants might be able to complete their entire life-cycle on a revolving 

 klinostat, as Phycomyces is actually able to do, in spite of the pronounced 

 influence of gravity upon its shape and direction of growth under normal 

 conditions. 



PART VI 



THE INFLUENCE OF CHEMICAL AGENCIES ON GROWTH 



SECTION 30. General. 



Beyond a certain optimal concentration every substance, whether 

 nutrient or not, exerts a retarding or inhibitory effect upon growth, due 

 either to the osmotic or to the poisonous action of the substance in question 3 . 



Within certain specific limits, however, plants may accommodate them- 

 selves to unusually concentrated nutrient solutions, while a deficiency of 

 food diminishes the activity of growth and of respiration, and may even 

 cause the plant to work more economically 4 . Not only food-substances, 

 but others also may influence growth. For example, sub-maximal doses of 

 various poisons frequently cause a transitory or even a prolonged increase 

 of respiration, and often of growth also 5 . An increase in the activity of 

 respiration or of protoplasmic streaming is, however, not necessarily accom- 

 panied by an increased activity of growth, for these results are due to a 

 complex of accelerating and retarding influences. 



1 Elfving, Verb. d. Grasknotens am Klinostaten, 1884 (repr. from Ofversigt af Finska Wetensk. 

 Soc. Forhandlingar, 1884); Barth, Geotrop. Wachsthumskriimmung d. Knoten, Leipziger Dissert., 

 1894, p. 32. 



2 [An organism all of whose constituents were of the same density as the medium in which 

 it floated would be absolutely irresponsive to and unaffected by gravity, so long as it was at rest, but 

 if it moved with constant velocity in irregular curves, every time the vertical component of its 

 velocity increased a gravitational force would act upon it, and when the vertical component decreased 

 this would be of negative sign. The actual velocity of movement of micro-organisms is however so 

 small that such influences are negligible.] 



3 On the influence of partial or complete starvation cf. Frank, Krankheiten der Pflanze, 1895, 

 2. Aufl., Bd. I, p. 278. 



4 On the effect of sudden changes cf. Reinhardt, Jahrb. f. wiss. Bot., 1892, Bd. xxm, p. 495; 

 Sokolowa, Das Wachsthum d. Wurzelhaare u. Rhizoiden, 1897, and the literature previously given. 



5 Ono, Die Wachsthumsbeschleunigung einiger Algen u. Pilze durch chemische Reize, 1900; 

 Journ. Coll. Sci. Imp. Univ., Tokyo. 



