THE MOVEMENTS AND SENSITIVITY OF PLANTS 329 



above ideas in their explanation of phototropic sensitivity. Many- 

 think that under the influence of the unilateral action of gravity, 

 special hormones are developed in the growing points, which may 

 iD crease or decrease the rate of growth. All these questions are 

 being studied actively at the present time, and it may be expected 

 that in the near future the information on the nature of tropisms, 

 as well as the general knowledge of growth phenomena, will be 

 extended. 



Geotropic as well as phototropic sensitivity is not to be con- 

 sidered as having constant properties. It may be influenced 

 by both external and internal conditions. Thus, for instance, due 

 to the effect of low temperature, negative geotropism of stems 

 may change to transverse geotropism, and then the shoot will begin 

 to grow in a horizontal position. This may be observed in spring in 

 many plants, which cling to the earth during the early cold period 

 and only later in the season begin to grow upwards. Similar 

 changes of 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 ethy- 

 lene per liter of air is sufficient to cause the seedlings of peas or 

 vetch to assume horizontal growth. In laboratories supplied with 

 gas, therefore, it is impossible to obtain normal seedlings of such 

 sensitive plants. 



Similar changes in geotropic behavior are effected also by 

 internal causes. Many rhizomes grow horizontally throughout 

 the summer, but produce a vertical shoot 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 in order to straighten the peduncle an 

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

 applied. However, towards the period of blooming its geotropic 

 : ' behavior " is changed and the peduncle straightens out, due to 

 the effect of the newly developed negative geotropism. The causes 

 which produce these changes of geotropic behavior are not under- 

 stood at present. 



96. Chemo-, Hydro-, and Thermotropism. — Besides the force 

 of gravity and light there are a number of other factors which may 

 cause curvatures in the growing organs of plants, if they are sub- 



