150 PLANT PHYSIOLOGY 



the effect of low temperatures, negative geotropism of stems, 

 for instance, may change to transverse geotropism, and then the 

 shoot begins to grow in a horizontal position. This may be 

 observed in many spring plants, which cling to the soil during 

 the early cold period and only later in the season begin to grow 

 upward. Similar changes in geotropic behavior may be seen 

 also when plants are subjected to the effect of certain gases, 

 ethylene and acetylene in particular. Plants appear to be 

 extremely sensitive to these gases. According to Neljubov's 

 investigations, 0.002 cc. of ethylene per liter of air is sufficient 

 to cause the seedlings of peas or vetch to assume horizontal 

 growth. In laboratories supplied with gas, it is therefore 

 impossible to obtain normal seedlings of such sensitive plants. 



Similar changes in geotropic behavior are produced also by 

 internal causes. Many rhizomes grow horizontally throughout 

 the summer but produce a vertical shoot during the following 

 spring. The peduncle of poppy exhibits positive geotropism 

 before the period of flowering, and therefore its tip is directed 

 downward. This is by no means caused by the weight of the 

 bud, as it was formerly supposed, for to straighten the peduncle 

 an amount of force far in excess of the weight of the bud has to 

 be applied. Toward the period of blooming, the geotropic 

 "behavior" of the peduncle is changed, and it straightens out, 

 owing to the effect of the newly developed negative geotropism. 

 The causes that produce these changes in geotropic behavior are 

 not understood at present. 



35. Influence of Light upon the Direction of Growth of Organs. 

 Phototropism. — Light is one of the factors indispensable to the 

 life of green plants, providing them with the energy required for 

 the decomposition of carbon dioxide. It is but natural therefore 

 that plants should display a high degree of sensitivity in relation 

 to light and should respond effectively to changes in the direction 

 and intensity of the light rays falling upon them. The ability 

 to react to the directive influence of light is called ''photo- 

 tropism." Young growing stems usually curve toward the source 

 of light, such a reaction being termed "positive phototropism" 

 (Fig. 40). Curvature in the direction opposite to incidence of 

 light is "negative phototropism," while the capacity of adjusting 

 organs, e.g., leaf blades, in a direction perpendicular to the 

 incident rays is known as "diaphototropism." As a rule, dorsi- 



