THE GENERAL CHEMISTRY AND PHYSICS OF PLANT-LIFE. 279 



more so than cells with micellse placed longituclinallj. Such 

 cells we have learned to kuow as the specific (dynamostatic) me- 

 chauical cells. Should these different cells occur on opposite 

 sides of au organ it will evidently result in bringing about a 

 curvature. The umbels of various Umbelliferce {Daucus, etc.) 

 show these different cell- structures and the resulting behavior 

 very clearly (O. Klein). 



As already indicated, there are cases in which the turgor of 

 living cells causes the expansion of tissue-layers, as, for example, 

 in the seed-coats of Impcdiens. 



To the categor}' of purely mechanical or hygroscopic move- 

 ments belong many teleological phenomena having no bearing 

 on reproduction, as, for example, the rolliug-in or folding of 

 leaves to guard against excessive evaporation, frequently notice- 

 able in plants of the desert (^Tschirch, Yolkens, and others). In 

 these cases the mechanical action producing the required move- 

 ment is also due to changes in the turgor of living cells or to 

 changes in the power of imbibition of the cell-membranes. Here 

 belon» the ynovement of the guard-cells of the breathing-jDores, 

 evidently due to changes in turgor (see the mechanics and an- 

 atom}'- of stomata). Finally, it should be remembered that the 

 purely mechanical movements may induce either simple curva- 

 tures or torsions. (The penetration of the seed of Erodium 

 gruinum into the soil is due to a process of torsion.) 



C. Autonomous Movements. 



These are due to internal causes and may recur periodically, 

 or they may occur only once or a few times during the life of the 

 plant (Pfeffer). To the autonomous movements which occur 

 only once or a few times belong the hook-like curvatures of 

 growing organs, the curvatures of anthers, and the unfold- 

 ing of floral envelopes. The above-mentioned circumnuta- 

 tion does not belong here, since this movement ceases when 

 the plant is placed upon the cliuostat (Baranetzky). Pfeffer," 

 however, considers it an autonomous movement. Accord- 

 ing to this author, autonomous nutations are perhaps more 

 or less present in all organs. Here we must also include 



Pflanzenpliysiologie, p. 184. 



