GEOTROPISM 165 



the floral organs although the latter are in most cases nearly or entirely 

 devoid of geotropic irritability l . 



Geotropism is in some cases of great importance in determining the 

 positions of plagiotropic main and' side shoots, but in other cases takes 

 little or no part in the orientation. The shoots of Lysimachia nummularia, 

 A triplex latifolia, and of Polygonum aviculare react plagio-geotropically in 

 strong light, but almost or entirely parallelo-geotropically in darkness, and 

 high and low temperatures may exert a similar effect. 



Foliage-leaves are very commonly plagio-geotropic, although in many 

 cases a special power of geotropic reaction is developed for particular 

 purposes. Thus in seedlings of Phoenix, Allium, and Yiicca the positive 

 geotropism of a portion of the cotyledon carries the radicle and axis of 

 the stem downwards into the ground 2 . According to Copeland 3 , the 

 hypocotyls of seedlings of Lupinus albus, Robinia psetid-acacia, Helianthus 

 annuus and Cucurbita Pepo act in the same way, owing to the fact that 

 their original positive geotropism soon becomes negative. 



The sporangiophores of Phycomyces nitens and Mucor mucedo are 

 strongly negatively geotropic 4 , whereas the mycelial hyphae of these fungi 5 , 

 as well as the stolons of Mucor stolonifer* show no perceptible geotropism. 

 The rhizoids of Bryopsis muscosa and of Caulerpa prolifera are positively, 

 the shoots negatively geotropic 7 . The same applies to Char a and Nitella 8 

 whose shoots show a fairly strong negatively geotropic reaction, as also do 

 the stalks of the perithecia of Xylaria carpophila, of Claviceps purpurea, and 

 the stalks of the sporophores of various of the larger Agaricineae 9 . The 

 lamellae, tubes, or lobes of the hymenium are, however, positively geotropic 10 . 

 Among Thallophyta in general, however, geotropism is less used for 



1 See the literature already given, and Wiesner, Sitzungsb. d. Wien. Akad., 1902, Bd. CXI, Abth. 

 i, p. 760. The downward bending of the fertilized flowers of Trifolium subterraneum and of 

 Arachis hypogaea, which causes the ripening fruits to be pushed into the soil, appears to be the result 

 of a change in the geotropic irritability. See Darwin, The Power of Movement in Plants ; Ross, 

 Malpighia, 1892, Fasc. VII-IX ; Huth, Ueber pericarpe, amphicarpe und heterocarpe Pflanzen, 1890. 



2 Sachs, Bot. Ztg., 1863, p. 59; 1862, p. 241 ; Copeland, Botanical Gazette, 1901, Vol. XXXI, 

 p. 410; Neubert, Jahrb. f. wiss. Bot., 1902, Bd. xxxvill, p. 119 (Allium). 



3 Copeland, 1. c. The stimulus appears in this case to be perceived by the root-tip. 



* Hofmeister, Pflanzenzelle, 1867, p. 286; Sachs, Arb. d. bot. Inst. in Wiirzburg, 1879, Bd. II, 

 p. 222; Wortmann, Bot. Ztg., 1881, p. 368; Dietz, Unters. a. d. bot. Inst. zu Tubingen, 1888, 

 Bd. II, p. 482 ; Steyer, Reizkriimmungen bei Phycomyces nitens, Leipzig. Diss., 1901, p. 6. 



5 Kny, Sitzungsb. d. bot. Vereins f. Brandenburg, 12. Juni, 1881 ; Steyer, I.e., p. 28. Kny (I.e.) 

 and Stammeroff (Flora, 1897, p. 148) found that pollen-tubes possess no geotropism. [They appear 

 also to be devoid of any heliotropic irritability.] * Wortmann, I. c., p. 384. 



7 Noll, Arb. d. Wurzburger Inst., 1888, Bd. Ill, p. 467 ; Klemm, Flora, 1893, p. 472. 



8 Hofmeister, 1. c., p. 286; Richter, Flora, 1894, p. 408. 



9 J. Schmitz, Linnaea, 1843, Bd. xvn, p. 474; Zopf, Die Pilze, 1890, p. 208. 



10 Sachs, Experimentalphysiologie, 1865, p. 93; Jahrb. f. wiss. Bot, 1863, Bd. in, p. 93. [The 

 stipes of Lentinus lepideus only become geotropic when the formation of a pileus has been induced 

 "by exposure to light. Buller, Ann. of Bot., 1905, Vol. XIX, p. 427.] 



