DTV. II 



PHYSIOLOGY 



345 



is not determined once and for all, but is liable to change owing to 

 internal and external influences. There is thus a " change of tone " 

 as regards geotropic stimulation. A certain " tone " is thus regarded 

 as the normal one, and the resulting reactions are expressed in the 

 distinction of orthotropous and plagiotropous, and positively and 

 negatively geotropic organs respectively. 



Among the external factors which influence the geotropic tone, 

 light, temperature, oxygen, and gravity itself may be mentioned, and 

 as an internal factor the developmental phase of the organ. 



The alteration of geotropic reaction by the illumination has an important 

 influence on the depth at which rhizomes occur. When the tip of a rhizome of 

 Adoxa growing on a slope becomes exposed to the light, its transverse geotropism 



1. 



FIG. 282. Rhizome of Polygonatum. The dotted Hue marks the surface of the soil. The aerial 

 shoots are cut off. Rhizome 1 was planted too high ; its continuation is downwards, only 

 the terminal bud which will form a flowering shoot being directed upwards. Rhizome 2 was 

 planted erect and too deep ; its continuation is obliquely upwards. (After RAUNKIAER.) 



becomes altered to positive geotropism, and this leads to the rhizome again enter- 

 ing the soil. Frequently the influence of light on the parts of the plant above 

 ground suffices to direct the subterranean rhizome. If the rhizome of Polygonatum 

 is planted too high in the soil, although covered by earth and in the dark, the 

 new growth turns obliquely downwards ; if planted too deeply it turns upwards 

 (Fig. 282). At the correct depth the rhizome is transversely geotropic. Light 

 also acts strongly on the geotropism of lateral roots ; when illuminated the lateral 

 roots of the first order approach the orthotropous position of rest much more 

 closely than they do in the dark. 



An effect of temperature may be observed on the stems of some spring plants ; 

 these often lie on the ground at temperatures in the neighbourhood of C. and 

 only become orthotropous at higher temperatures. With lack of oxygen many 

 roots and rhizomes become negatively geotropic, and thus reach regions where more 

 oxygen is available. 



Changes of tone due to internal causes are seen, for instance, in rhizomes, which 



