IRRITABILITY. 437 



the conditions which are essential to growth in length. 

 Under the exceptional conditions of illumination in the case 

 before us, growth in length is rendered impossible ; but the 

 organ is nevertheless essentially a growing organ, for it is 

 endowed with all the properties which, as we learned in a 

 previous lecture (p. 341), are characteristic of growing organs. 

 Amongst these properties, that of irritability was found to be 

 conspicuous, and it is manifested in this case by a curvature 

 which is not accompanied by growth in length. 



We may now conveniently enquire as to what part of the 

 growing organ is the seat of the heliotropic curvature, or, in 

 other words, to ascertain the distribution of the heliotropic 

 curvature throughout the growing region. Miiller-Thurgau 

 found, in the case of certain positively heliotropic stems, that 

 curvature was most evident in the most rapidly growing 

 zone, and he further found that this was also the case in the 

 negatively heliotropic aerial roots of Monster a Lennea and of 

 Chlorophytum. Wiesner has carefully investigated this point 

 and concludes from his observations that, in stems which are 

 only moderately sensitive to the heliotropic action of light, 

 the greatest curvature takes place in the most rapidly growing 

 zone, whereas in stems which are very sensitive the greatest 

 curvature does not take place in the most rapidly growing 

 zone. In the latter case the seat of greatest curvature is 

 sometimes above and sometimes below the zone of most rapid 

 growth; when the organ is young the former is the case, 

 when it is older, the latter. The form of the curved organ 

 will, of course, vary under these different circumstances. 



It will be observed that in all cases heliotropic curvature 

 is limited to the growing region, or, to put it more generally, 

 to the region which is capable of growing. This is illustrated 

 in an interesting manner by the behaviour of shoots, such as 

 those of Caryophyllaceae, Grasses, etc., in which the nodes 

 are well-marked and the leaves sheathing. In these shoots 

 the tissue at the lower end of each internode, surrounded 

 by the sheathing leaf-bases, remains capable of growth (see 

 p. 333). If now light is allowed to fall upon one side of 

 such a shoot, it curves heliotropically, the curvature taking 



