664 PLANT MOVEMENTS 



revealed that few species exhibited positive hydrotropism. In most species 

 the roots that started to grow downward as a consequence of gravitational 

 attraction soon ceased to grow because of insufficient water. Roots that hap- 

 pened to be produced in the moist soil grew normally. Positive hydrotropism 

 was found to be present in a few species of the Cucurbitaceae and Leguminosae 

 since the roots of seedlings of these species curved away from the dry sod 

 toward the moist soil and grew along the boundary between the moist and 

 dry soil. The experiments demonstrate that hydrotropic curvatures do occur 

 in the soil grown roots of some species but also indicate that such curvatures 

 are probably not a common phenomenon under field conditions. 



Hydrotropic curvatures are caused by differences in the rate of enlarge- 

 ment of the cells on the opposite sides of the root but no satisfactory explana- 

 tion of the cause of this unequal growth has been suggested. 



Traumatotropism. — Injury to the tissues of plants, especially to stems 

 and roots, often results in curvatures caused by unequal growth. Such curva- 

 tures are examples of traumatotropic reactions. Transverse incisions in oat 

 coleoptiles commonly result in positive curvatures but decapitated coleoptiles 

 usually fail to react to injuiT inflicted near the upper end. Incisions made 

 near the base of decapitated coleoptiles may induce positive curvatures, how- 

 ever. From the evidence available it seems probable that injuries influence 

 either the distribution of auxin in the tissue (Keeble and Nelson, i935) 

 or interfere with the transport of foods or other substances into the growmg 

 regions from other parts of the plant, but no more detailed analysis of the 

 mechanism of these reactions is possible in terms of currently available m- 

 formation. 



Nastic Movements. — The essential distinction between nastic and tropic 

 movements has already been described. With few exceptions, nastic move- 

 ments are evoked by environmental factors such as temperature or diffuse 

 light which influence the organ with equal intensity from all directions, while 

 tropic movements are induced by factors which act with greater intensity 

 from one direction than others. Equal illumination of all parts of a plant 

 organ, for example, may result in photonastic movement, causing the organ 

 to assume a different position in the light than in the dark. Photo tropic 

 movement of a plant organ occurs, however, only when the organ is illuminated 

 unequally from different directions. Furthermore, nastic movements are 

 chiefly restricted to organs such as leaves and petals, the structure of which 

 largely or entirely prevents their movement except in certain directions. 



The leaves of a number of different species of plants undergo marked 

 changes in position during the day and night. In some species the leaves 

 droop at night and become oriented more or less horizontally during the day. 



