GEOTROPISM 163 



Fungi, which are aided by this means to develop their spores in air where 

 dispersal is possible but which do not primarily need illumination. 



The lateral roots of the third or fourth order, thorns, hairs, and the stems 

 of the Mistletoe, are nearly or entirely devoid of geotropic and heliotropic 

 irritability, and hence grow in all directions independently of the direction 

 of gravity and of the illumination. The presence of a geotropic irritability 

 in a fungus mycelium might even become injurious by causing it to pass 

 from a suitable medium to comparatively innutritive soil. 



The fact that the perpendicularity of the main axis is determined 

 by gravity is at once shown when a seedling is laid horizontally, for the 

 growing zone of the root curves downwards, and of the stem upwards 

 (Fig. 35). The lateral parts of the first order possess a definite diageo- 

 tropism, since they assume much the same angle with the perpendicular 

 whether the main root is laid horizontally or is even placed upside down 1 . 

 The same fact shows that they are radial organs, and that directive influences 

 radiating from the main root exercise little or no effect upon them. In 

 all experiments of this kind it is natu- 

 rally essential that the conditions 

 should be kept as constant as possible, 

 and in this case the geotropic response 

 of the lateral roots is dependent not 

 only upon the intensity of the stimulus 

 but also upon the external conditions 

 and the tone of the root. The lateral 

 roots arising from the hypocotyl and 



base Of the main rOOt Often grOW after being laid horizontally. 



more or less horizontally as the result of their diageotropism, whereas later 

 roots arising at the base may form angles of 80 to 60 or even of 45 

 with the perpendicular. In order that the root-system may spread 

 thoroughly through the soil it is necessary that the geotropic irritability 

 of side roots of the second and third order should diminish ; and in fact, 

 according to Sachs, the roots of the second order of Zea Mays have only 

 a feeble, and those of Cucurbita Pepo no geotropic irritability. It does 

 not, however, follow that lateral axes are always less irritable geotropically 

 or heliotropically than the main axis, for we are dealing here with special 

 phenomena of accommodation. 



1 Dutrochet (Rech. s. la structure d. animaux et d. vegetaux, 1824, p. 102) supposed the direction 

 of the lateral roots to be determined as the resultant of their geotropism and their tendency to set 

 themselves at right angles to the main root. The matter was more fully explained by Sachs, Arb. 

 d. bot. Inst. in Wiirzburg, 1874, Bd. I, p. 602. Cf. also Czapek, Sitzungsb. d. Wiener Akad., 1895, 

 Bd. civ, Abth. I, p. 1197; Jahrb. f. wiss. Bot., 1895, Bd. xxvn, p. 328; 1898, Bd. xxxn, p. 247 ; 

 Schober, Bot. Ztg., 1898, p. I ; Guillen, Compt. rend., 1901, T. cxxxil, p. 589. 



M 2 



