IRRITABILITY. 5OI 



marked, but I am not aware that any comparison of their 

 heliotropic with their geotropic irritability has as yet been 

 instituted. 



We pass now to plagiotropic organs. The oblique direction 

 of growth is, as we have seen in previous lectures, assumed by 

 both radial and dorsiventral organs, including shoots, roots, 

 and leaves. 



The simplest cases of plagiotropism are those offered by 

 radial organs, such, for instance, as certain rhizomes, and 

 lateral roots. Elfving observed, namely (p. 462), that certain 

 rhizomes (Heleocharis palustris^ Sparganium ramosum, Scirpus 

 maritimus), grow horizontally beneath the surface of the soil. 

 The only external directive influence, apparently, which de- 

 termines their direction of growth is that of gravity, and their 

 irritability to its action is of the kind with which we have 

 become acquainted as Diageotropism. The same behaviour 

 has more recently been observed by Stahl in the rhizomes of 

 Adoxa mosckatellina, Circcea lutetiana, and Trientalis europcea. 



Though light takes no part, under ordinary conditions, in 

 determining the direction of growth of these rhizomes, yet 

 their behaviour when exposed to light is of interest. Stahl 

 has observed that when a rhizome of Adoxa is exposed to 

 light, it curves so that its direction of growth becomes either 

 obliquely or vertically descending. He explains this change 

 in the direction of growth by assuming that exposure to 

 light alters the nature of the geotropic irritability of the 

 rhizome, so that from being diageotropic it becomes positively 

 geotropic. The downward curvature is clearly not due to 

 negative heliotropism, for Stahl has conclusively proved that 

 the rhizomes do not respond to the directive influence of light. 

 This is a point which merits further investigation, for it is 

 difficult to conceive that exposure to light should thus modify 

 the geotropic irritability. In view of what we know as to 

 the influence of light upon the physiological properties of 

 the shoots of Tropaeolum and of the Ivy (p. 425), it would 

 seem not impossible that exposure to light induces dorsi- 

 ventrality in the rhizomes, and leads to photo-epinasty. If 

 this be so, then the downward direction of growth of the 



