NATURE OF PLANTS 33 



cell sap. Red cells often indicate acidity and they become blue 

 in weak potash solutions. So violet and blue indicate an al- 

 kaline condition and such cells change to red in weak acid solu- 

 tions. This is essentially the explanation of the changing tints 

 in the autumn. The changing character of the cell sap is at- 

 tended with a gradual decomposition of the complex chlorophyll 

 and other materials that results in the formation of a series of 

 substances that are characterized by a variety of colors. Here 

 again the matter of coloration has been over emphasized. While 

 nitrogen, phosphorus and potassium are transported from the 

 leaves before they fall, the coloration does not effect so complete 

 an emptying of the leaves as was previously believed, if indeed it 

 is of any service. 



15. The Cause of Leaf Arrangement. — How is the perfection 

 in the arrangement of the leaves accomplished? The leaf or 

 any other organ does not remain in a fixed position during its 

 growth and development but owing to the fact that first one side 

 and then an adjoining side of the organ is growing faster than 

 any other part it comes about that the organ is bent from side to 

 side by the more rapidly growing cells and the apex of the organ 

 is often caused to travel through a rather irregular circle. These 

 growing organs are sensitive to light, gravity, moisture and other 

 stimuli. As a result of these movements the organ is brought 

 into relations with various intensities of light, heat, etc. In cer- 

 tain positions the stimuli are not favorable and they cause it to 

 grow away from this position while in other positions the stimuli 

 act in the most favorable way and the organ is stimulated to its 

 best growth. So during the development of the organ it is 

 brought into varied relations with the forces which affect it and 

 as a result of this experience the leaf or other organ is directed 

 by the stimuli and at maturity finally comes to rest in a fixed 

 position that is the most advantageous. The leaf ordinarily so 

 reacts to these stimuli that its blade is exposed at right angles 

 to the rays of light, but in the iris, many grasses and rushes 

 the blades are nearly erect. Quite a large number of plants bear 

 their leaves edgewise. The giant trees of Australia {Eucalyptus) 

 and the so-called compass plants are familiar examples (Fig. 

 3 



