322 PHYSIOLOGY OF GROWTH AND CONFIGURATION 



and the petiole frequently falls. This phenomenon is called nyctitropism or 

 night movement of the leaves. 6 The falling of mechanically stimulated leaflets 

 of Mimosa is caused by decreased turgidity in the lower half of the pulvinus, but 

 the night movement of Mimosa leaves is the result of increased turgidity in the 

 upper half of the pulvinus. Artificial shading also causes nyctitropic move- 

 ments in leaves. In Fig. 160 two branches of Desmodium gyrans are shown, 

 the leaves of one in the day position and those of the other in the night position. 

 The approach of night causes the leaves to fall and lie against one another. 



Summary 



i. General Survey of Plant Movements. — Bendings of ordinary plants (move- 

 ments of one part of the plant body with reference to the rest) may be classified in 

 two groups, growth movements and variation or turgor movements. Each group has two 

 classes, autonomic and paratonic movements. Autonomic movements, whether of 

 growth or of variation, are specifically controlled by internal conditions; that is, they 

 are automatic or spontaneous, not being occasioned by changes or one-sided influences 

 in the surroundings. Paratonic movements, of both primary groups, are always 

 occasioned by some special change or asymmetry in the surroundings. For autonomic 

 movements the development of the plant first prepares the mechanism for movement 

 and then supplies the stimulus that sets the mechanism in operation. For paratonic 

 movements the developmental processes prepare the mechanism, but they do not 

 furnish the stimulus that sets the mechanism in operation; the stimulus must come 

 from the environment. 



Growth movements are due to unequal enlargement on the opposite sides of the 

 bending part. Here belong the bendings due to the tropisms. They are always 

 confined to parts that are still in the second phase of growth, the phase of enlargement. 

 Variation movements occur in mature or maturing parts (that have ceased to enlarge). 

 They are due to rapid changes in the turgidity of certain portions or regions of the 

 bending organ, which set up new kinds of tissue strains and thus produce bending. 



Some of the main paratonic growth movements have been considered (Chapter III, 

 Sections 5 and 6), those due to phototropism and geotropism. The autonomic growth 

 movements include nutation, circumnutation, the circular movements of twiners, and 

 the nastic bendings. Epinasty is more rapid enlargement on the upper side of a leaf, 

 resulting in a downward bending (as in dandelion leaves). Hyponasty is more rapid 

 enlargement on the lower side of the organ, resulting in an upward bending. 



The following scheme will help to make the classification of plant movements more 

 easily understood. 



I. Growth movements (confined to enlarging parts or organs). 

 1. Autonomic growth movements (due to internal stimuli). 



(a) Nutation (as of bean epicotyls). 



(b) Circumnutation (as of most vertically elongating shoots). 



(c) Twining movements (as of the terminal internodes of hop, etc.). 



(d) Nastic bendings (epinasty and hyponasty). 



b These movements are not properly to be considered as tropisms, for the latter are all growth 

 bendings. Perhaps the best term for the day-night movements in question (which are para- 

 tonic movements of variation) is photeolic movements, due to alterations in light intensity. 

 — Ed. 



