3°° 



PHYSIOLOGY OF GROWTH AND CONFIGURATION 



tion (Fig. 146, A), but forms thin hyphse with pointed branches in peptone 

 solution (Fig. 146, B). 1 



The hay bacillus (Bacillus subtilis) shows pronounced polymorphism, ac- 

 cording to the medium in which it grows. 2 In a slightly alkaline, 5-per cent, 

 solution of beef-extract the cells are rod-shaped, 6-10 /j, long and 0.5 n in diam- 

 eter (Fig. 147, 1, a). In neutral, 5-per cent, sugar solution, containing also 0.1 

 per cent, of beef-extract, the cells are shorter and thicker, 4-6 /x long and 0.8 /x 

 in diameter (Fig. 147, 2, a). Very large cells are produced in hay infusion, 12 jx 

 long and 1.0 /jl in diameter (Fig. 147, 3, a). In all of these media cell division 

 proceeds very rapidly, but the newly-formed cross-walls are so thin and so 



little refractive toward light that they cannot 

 be seen at all excepting in stained preparations. 

 When the rods above described are stained 

 with iodine, each one is seen to be composed of 

 a chain of much shorter cells (Fig. 147, 1, b; 2, 

 b; 3 ,b).* 



§8. Influence of Wounding, Traction and 

 Pressure on Growth and Configuration. — 

 Wounding of all sorts exerts a pronounced in- 

 fluence upon the rate of growth of plant 

 organs; a wound may simply retard growth or 

 may cause it to cease altogether. Wounding 

 is frequently followed, also, by various kinds 

 of bendings in growing organs. Especially 

 noteworthy is the Darwinian response of roots, 

 so called by Wiesner, in honor of Charles 

 Darwin, 4 who first described this reaction. If 

 a root-tip is laterally wounded (as by cutting, 

 burning, etc.), the root bends, after a time, in 

 the direction toward the uninjured side. 

 Frequently this bending is so pronounced that 

 the root-tip is carried upward and then 

 downward again, thus forming a loop in the growing region. This response 

 has sometimes been regarded as purposeful, since its effect is to remove 

 the root-tip from a dangerous neighborhood. Wiesner 5 has shown, in later 

 studies, that the Darwinian response is really a double one, being composed of 

 two consecutive bendings in opposite directions. From twenty-five to forty- 

 five minutes after the occurrence of the wounding a very slight bending takes 

 place in the upper portion of the region of growth, the direction of this bending 

 being such as to move the root-tip toward the position occupied, at the time of 



Fig. 148. — Lupine seedling with 

 bent primary root, showing the 

 formation of laterals exclusively on 

 the convex side of each bend. 



1 Klebs, Georg, Die Bedingungen der Fortpflanzung bei einigen Algen und Pilzen. Jena, 1896. 

 2 Buchner, Hans, Beitrage zur Morphologie der Spaltpilze. Nageli's Untersuchungen fiber niederen. 

 Pilze aus dem Pflanzen. Miinchen and Leipzig, 1882. P. 205-224. 



3 Also, compare the experiments of Ritter, 1907. [See note 1, p. 270. | 

 1 [Darwin and Darwin, 1880. [See note 1, p. 314.] 

 » Wiesner, 1884. [See note 3, p. 292.] 



