MOVEMENTS OF IRRITATION. 475 



length is divided by ink-lines into zones of 2 mm. length. 

 Measurements of the growth made in the usual way show that 

 the tip of the plumule only grows very slowly; the rate of growth 

 increases as we proceed downwards, attains its maximum say in 

 the fourth or fifth zone, and then again falls off. Hence it is 

 clear, and this is undoubtedly interesting, that the zone of greatest 

 heliotropic perceptivity (the tip) is by no means at the same time 

 that of greatest growth also. 



If perfectly straight seedlings of Avena, grown in the dark, are 

 decapitated by removing from their tip with a sharp transverse 

 cut a zone 3 or 4 mm. long, and are then illuminated unilaterally, 

 their growth is only slow, and heliotropic nutations are not per- 

 formed at all. After a few hours, however, the rate of growth of 

 the sheath leaf again becomes more rapid, and heliotropic sen- 

 sitiveness also returns.* Many of the experiments described are 

 also of great significance, because they teach that the power 

 which the organs possess of reacting and their power of per- 

 ceiving (their irritability and sensitiveness respectively) are to 

 be regarded as two distinct things. 



1 See Miiller-Thurgau, Flora, 1876. 



2 See Oltmanns, Flora, 1892, p. 223. 



3 See Rothert, Ber. d. Deutsclien botan. Gesellscli., Bd. 10, and Cohn's Beitrdge 

 zur Biologie d. Pflanzen, Bd. 7. This latter research I have unfortunately been 

 unable to consider further. 



180. The Hydrotropism of Roots. 



When the moisture in the medium in which roots are develop- 

 ing is not uniformly distributed the roots execute hydrotropic 

 curvatures, and following Sachs' method 1 it can readily be de- 

 termined that roots are positively hydrotropic. The apparatus 

 necessary for demonstrating the phenomena here under considera- 

 tion is shown in section in Fig. 160. , A hoop of strong sheet zinc 

 about 5 cm. high and 20 cm. in diameter is covered with wide- 

 meshed netting, so as to form a sieve, of which the porous bottom 



* The decapitation at first completely destroys the heliotropic perceptivity of 

 the seedlings ; they consequently do not curve, although growth is slowly pro- 

 ceeding. Later on a fresh " physiological apex " is to some extent developed. 

 The seedlings are then almost as irritable as absolutely uninjured ones. 



