THE PLANT BODY 



29 



Petiol 



Petiole 



In the case of roots the older portions of the root systems are 

 already fixed in their position by the surrounding solid earth; 

 but the new roots which grow from the old ones each spring 

 are enabled to move and penetrate into new soil regions in 

 order to absorb foods and water. In a similar manner each new 

 annual crop of leaves can adjust itself to the conditions con- 

 fronting it in the season in which it must do its work. Flow- 

 ers are likewise able to adjust their positions so that they 

 may secure the visits of pollinating insects, and fruits assume 

 positions suitable for dissem- 

 ination by wind or animals. 

 The older parts, which have 

 lost the power of growth, and 

 therefore of movement, thus 

 become supporting, storing, 

 and conducting organs of 

 plants, while the annual 

 growth of new roots, leaves, 

 and flowers enables the or- 

 ganisms to adjust them- 

 selves to any ordinary change 

 which may take place in 

 the environment, and thus to adapt themselves" to the usual 

 seasonal changes and other requirements of their surroundings. 



Special motor organs exist in some plants, however, which 

 enable certain organs to execute more rapid and exact move- 

 ments than those described above. Examples of such motor 

 organs are found in the leaves of peas and their near relatives. 

 These special organs are termed pulvini (singular, pulvinus) 

 (Figs. 15 and 16). They are highly modified portions of the 

 petioles at the base of each leaflet arid often at the base of 

 the petiole itself, where it joins the main stem or a branch. 

 The quick curvature of these pulvini serves to swing the leaves 

 into positions which enable them to secure a better adaptation 

 to light or a more complete protection from too great loss of 

 water. Internal and unknown stimuli also affect plant move- 

 ments, in some instances quite as profoundly as the external 



FIG. 16. Pulvini of the bean 

 Magnified. After Sachs 



