144 



PLANT PHYSIOLOGY 



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10 864 

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the phenomenon of growth, and for this reason the curvatures 

 of growing organs have been designated the "growth move- 

 ments" of plants. There may be distinguished two types of 

 movements: (1) tropisms produced by directive stimuU, e.g., 

 gravity or light; and (2) nasties, curvatures produced by stimuli 

 that affect the whole plant uniformly and do not determine the 

 direction of response, e.g., changes in temperature, humidity 

 of the medium, or the general illumination. Growth movements 



are the result of the unequal 

 growth of tissues on the different 

 sides of the bending organ. The 

 side that grows faster becomes 

 convex, while the side that is 

 slower in its growth becomes con- 

 cave ; and the organ curves to the 

 side on which growth is slower. 

 Such movements of course may be 

 observed only in organs growing 

 rapidly, or at least in organs that 

 have not stopped growth. 



This relation may be easily 

 observed in the following simple 



Fig. 87.^Geotropic curvature's of experiment. If Seedlings, e.g., of 

 i)ean roots in humid air. A, the peas or beans, are placed horizon- 



r„trc,''24"hrL1er^'Th"e^dtut'iI: tally, after a period of a few 



angle is a paper mark fixed on glass hourS the stemS will turn Upward, 



{after Sachs). ^^^^^e the root will bend forcibly 



downward. If these organs are previously marked at intervals 

 with India ink, as is done for the purpose of studying the rate 

 of growth, it will be noticed that the greatest curvature occurs 

 in the region of the most rapid elongation. The fully grown 

 parts do not manifest any tendency to curve, remaining in the 

 same position in which they were placed (Fig. 37). 



A seeming exception to this general rule are the culms of 

 cereals, which are capable of rising after ''lodging," by forming 

 a bend just above the node (Fig. 38). This phenomenon, which 

 is of great importance in saving lodged grain, is due to the fact 

 that the stem tissues above the nodes of cereals retain the ability 

 to renew growth for a considerable time. As soon as the stem 

 has been bent into a horizontal position, the lower side of the 



