654 PLANT MOVEMENTS 



another. The direction of the curvatures resulting from such movements 

 often bears a relation to the direction from which the initiating factor acts 

 with greatest intensity. The curvatures of growing stems and roots induced 

 by differences in the intensity or quality of incident light {phototropic curva- 

 tures) and those resulting under the influence of gravity {gcotropic curva- 

 tures) are the most familiar tropic movements. Similarly changes in the 

 position of organs which are evoked by differences in the water content of 

 the soil, by physical contact, and by specific chemical compounds are known 

 as hydrotropic, thigtnotropic, and chemotropk movements respectively. The 

 movement is considered to be positive when the organ bends toward the direc- 

 tion from which the factor is acting and negative when it is in the opposite 



direction. 



Nastic movements are those that occur in plant organs when the initiating 

 factor affects all parts of the growing organ uniformly, or when the initiating 

 factor, although acting entirely or principally from one direction, evokes a 

 reaction which occurs in the same manner and in the same direction regard- 

 less of the direction from which the factor acts. The growth movements of 

 very young leaves, bud scales, and flower petals are examples of nastic move- 

 ments. At first the morphologically lower side of these structures grows more 

 rapidly than the upper side so that they are bent upward and enclose the 

 stem tip. This more rapid growth of the lower side is known as hyponasty 

 {hypo=^\owcr). The opening of the bud is brought about by a more rapid 

 growth of the morphologically upper side of these structures, a phenomenon 

 known as epinasty {c pi = upper). 



Although the main stem of most herbaceous plants appears to grow straight 

 upward careful measurements demonstrate that the stem tip actually traces 

 an irregular spiral pathway in space as it elongates. This approximately 

 spiral movement of growing stem tips is known as nutation. Nutation re- 

 sults from unequal rates of growth in different vertical segments around the 

 stem axis and often seems to occur independently of environmental factors. 

 The term nutation is sometimes used to include all movements which result 

 from unequal rates of growth. In this book, however, the term will be used 

 only in the more limited sense just specified. 



2. Turgor Movements. — Movements of plant organs caused by reversible 

 changes in cell volume are known as turgor rnovements. Many of these move- 

 ments are initiated in compact groups of relatively large, thin-walled cells 

 that constitute the so-called "motor organs" or pulvini but they may also 

 occur in any tissue that is largely composed of living, thin-walled cells. Ex- 

 amples of turgor movements associated with pulvini are the spectacular reac- 

 tions of the sensitive plant {Mimosa pudica) to slight shocks or other 



