Growth Movements 505 



mosse. All plants possessing jointed leaves do not exhibit 

 the same behavior. Nyctitropic movements are com- 

 monly due to changes of turgidity, and growth is not 

 usually involved. The articulations are cushions in which 

 cortical tissue predominates. Under stimulation the 

 dorsal and ventral halves give osmotic changes unequal 

 in rapidity so that movement is brought about. 



LABORATORY WORK 



Geotropism. - - For a few observations upon the geotropism of 

 roots fairly large seeds are desirable, such as those of peas or 

 beans. Germinate the seed in moss or on paraffined wire netting 

 over water. When germination has progressed to the extent of 

 a few centimeters, the roots may be marked off with India ink 

 as for determining the region of extension. The growth curva- 

 tures are then to be followed by placing the radicles in a hori- 

 zontal, or any other desired, position. If only a few seed are 

 used, they may be pinned to the lower side of large corks covering 

 jars or dishes partially filled with water. 



For a larger number of seed and particularly for observations 

 respecting the effects on side roots, the seedlings may be arranged 

 at various angles on two thicknesses of moistened carpet or felt 

 paper between plates of glass. The plates are clamped together 

 with wooden clothespins, and wads of filter-paper here and there 

 prevent crushing. Place the plates on edge in a moist greenhouse 

 or cover with wet cloths. Observe from time to time, note the 

 results, and shift the position of the plates through ninety degrees 

 after secondary roots are produced. Discuss the results. 



Negative geotropism of young shoots may be followed by ob- 

 serving the behavior of bean or pea seedlings when the pots are 

 placed horizontally. Determine also the time of presentation and 

 of reaction for such seedlings grown in very small (2 inches) 

 pots. Compare the presentation time at 12 to 15 with the 

 interval at 25 to 30 C. 



Secure shoots of Tradescantia or of oats embracing several 



