281 SUMMARY OF CURRENT RESEARCHES RELATING TO 



the tendency to grow not in a straight line, as is the case with or- 

 dinary orthotropous internodes which do not twine, but in a spiral, 

 which is very flat at the apex from the combination of the two forces, 

 but becomes gradually steeper and steeper towards the base. 



If a stem growing in such a spiral meets with no support, but is 

 at the same time protected from falling, it will, after its growth in 

 length is completed, assume a straight and vertical position, like any 

 other orthotropous stem, since the spiral line becomes constantly 

 steeper towards the base so long as growth continues and geotropism 

 is also acting. This property enables the strongest twining plants 

 to cling round the slenderest supports. The true object of the 

 support is to act as a hindrance to the straightening of the stem 

 which is growing with a spiral movement. The stoppage of growth 

 by thick supports causes the internodes of stems which coil round 

 them to be usually shorter than those of stems coiling round slender 

 supports. The erect position of the higher internodes prevents the 

 terminal bud ever being at a great distance from the support. The 

 torsions so often observed in climbing plants are of secondary 

 importance in the process of coiling. 



Mechanism of Twining.* — Herr H. Ambronn discusses all the 

 previous theories on this subject, and confirms Baranetzki's statement 

 that the circumnutation of twining plants ceases when they are made 

 to rotate slowly round a horizontal axis. He considers the movement 

 of twining to be made up of three factors, viz. (1) circumnutation ; 

 (2) negative geotropism ; (3) the resistance offered by the support to 

 the movements of the apex of the shoot. The part played by these 

 three factors is discussed in detail. 



Sensitiveness to Contact. f — Herr W. Pfeffer insists on the dis- 

 tinction between sensitiveness to contact and sensitiveness to impact 

 (" Stossreize "). The first is the result of continuous contact with a 

 solid body, as in the case of tendrils; the second of momentary 

 powerful action, as in the sensitive plant. Static pressure does not 

 bring about the second kind of irritation when it is unequal, and 

 consequently causes unequal pressure on neighbouring points. This 

 was proved by the pressure of water, mercury, and gelatin. In the 

 case of tendrils the weight of the body is no factor in the sensitive- 

 ness. Pieces of cotton-wool of the weight 0-00025 mgr. produced 

 no effect if carefully placed on the tendril, but did when they caused 

 gentle impact by slight currents of air. 



Contrary to the statement of Darwin, the author found the 

 glandular hairs (tentacles) of Drosera to have a sensitiveness very 

 similar to that of tendrils, statical pressure producing no effect. 

 Small pearls or splinters of glass only produced irritation of the 

 glands when they caused a rubbing as the result of concussion ; fluids 

 producing no efiect, as with tendrils. 



* Ber. Math.-Phys. Klasse K. Sachs. Gesell. Wiss. Leipzig, 1885. See Bot. 

 Centralbl., xxiv. (1885) p. 81. 



t Unters. aus d. Bot. lostit. Tubingen, i. (1885), Heft 4. See Bot. Centralbl., 

 xxiv. (1885) p. 75. 



