CHAPTER V 



MOVEMENTS OF VARIATION" 



§i. General Survey of Plant Movements. — All characteristic plant move- 

 ments may be classified into two groups: the first includes growth movements, of 

 growing organs, including nutation movements and tropisms, while the second 

 embraces movements of mature organs, movements of variation. Only growth 

 movements (the first group) have thus far been considered; as has been seen, 

 they occur in growing organs and cease when growth is completed. Growth 

 movements may, in their turn, be separated into two groups: in one are included 

 movements that are produced by external conditions, such as movements due 

 to phototropism and geotropism, which are called par atonic or receptive growth 

 movements; in the other group are included autonomic or spontaneous growth 

 movements, which are dependent upon the internal organization of the plant, 

 such as the circular movements of twiners; and those due to epinasty, hypo- 

 nasty, etc. Epinasty is the phenomenon of increased growth on the upper 

 side of a leaf or stem, as compared to the lower side, and it results in the 

 downward bending of the organ. Hyponasty denotes the opposite condition, 

 where growth is more rapid on the under side of an organ. Both phenomena 

 depend upon the internal organization of the organs in question, rather than 

 upon specific stimuli from the surroundings. Movements of variation are 

 also divided into paratonic and autonomic movements. 



§2. Autonomic Movements of Variation. — Among the somewhat limited 

 number of cases of autonomic movements of variation that are now known, the 

 most striking example is found in the lateral leaflets of Desmodium gyrans, 1 

 the free ends of which move through an elliptical path. The rate of movement 

 depends upon the temperature; with high summer temperature a complete 

 circuit is completed in about three minutes. Similar, but much slower, move- 

 ments may be observed in other plants; thus, the terminal leaflet of red clover 

 (Trifolium pratense) completes its upward and downward movement in from 

 one to four hours at summer temperatures. 



§3. Paratonic Movements of Variation. 2 — The leaves of Mimosa pudica, 

 (sensitive plant) which droop at a slight touch, furnish the best example of 

 paratonic movements of variation. The leaf consists of a long petiole to which 



1 Hofmeister, Wihelm F. B., Die Lehre von der Pflanzenzelle. Leipzig, 1867. 



2 Briicke, Ernst, Ueber die Bewegungen der Mimosa pudica. Muller's Arch. Anat. Physiol, u. wiss. 

 Med. 1848: 434-455. 1848. Pfeffer, 1873. [See note 2, p. 291.] Haberlandt, G., Reizleitendes Gewebe- 

 system der Sinnpflanze. Leipzig, 1890. [MacDougal, D. T., The mechanism of movement and trans- 

 mission of impulses in Mimosa and other "sensitive" plants; a review with some additional experiments. 

 Bot. gaz. 22: 293-300. 1896.] 



In general, see: MacDougal, 1 901. [See note 1, p. 311.] Pringsheim, 1912. [Seenotei, 

 p. 253].— Ed. 



316 



