SPECIAL CASES 413 



grasses, are purely physical in origin. Whereas in the first-named cases 

 not only does the plant see to the storage of the required potential energy, 

 but also so regulates matters that the tensions are released spontaneously 

 or by an external agency at a definite time. Since the contracting tissues 

 are never perfectly elastic, the full amount of the stored energy can never 

 be used in propelling the seeds '. 



Mechanisms of this kind are only capable of a single response, whereas 

 the regeneration of the tissue-tensions in the stamens of Cynareae, and in 

 the pulvini of Mimosa^ renders frequent repetition possible. A sudden fall 

 of turgor allows the elastic walls to contract, the restoration of turgor 

 redistends them, but whether changes in the elasticity of the cell-wall may 

 also occur is uncertain. From a mechanical point of view the mode in 

 which turgidity is restored is immaterial, and the escape of water is the 

 result of the fall of turgor, so that sudden contraction can only take place 

 when a rapid filtration of water under pressure through the cell-wall is 

 possible. 



The energy of contraction is as great in these motile tissues as in 

 animal muscle, in which it may be from i to 10 kilograms per sq. cm., while 

 a load of 5 kilograms per sq. cm. is required to prevent a staminal filament 

 of one of the Cynareae from contracting. In both cases most work is done 

 when the load is such that contraction is just possible, and to get the full 

 contraction the load must be steadily decreased as contraction continues 2 . 

 Otherwise an excessive load at any phase of contraction prevents the 

 shortening and hence also prevents work from being done. In precisely 

 the same way the maximal work is done during the subsequent elonga- 

 tion of the filament, if a resistance is interposed of sufficient intensity to 

 prevent movement until turgor is fully restored, and if the filament is then 

 allowed to elongate to its full extent by gradually removing the resistance. 

 A growing organ, on the other hand, which exerts a constant pressure upon 

 a resistance pushed in front of it, performs the same amount of work in 

 unit time so long as the rate of growth remains the same. 



1 Pfeffer, Studien zur Energetik, 1892, p. 239. 



3 Pfeffer, 1. c., pp. 236, 238. The same applies to the work done during the expansion of a com- 

 pressed gas, or the contraction of a rarefied one. It is uncertain whether the slight increase in the 

 production of heat in the pulvinus of Mimosa pudica during a movement produced by stimulation 

 is due to a chemical reaction, or to the internal friction produced as the water escapes through the 

 cell-sap. [The latter is hardly probable. Suppose a total of 5 gram, centimetres of work were 



done, a high estimate, this would represent - t of a calorie. If the region warmed corresponded 



to 5 mg. of water, it would be raised only ^ C. in temperature, even if all the heat was retained 

 during the whole time of contraction. It must further be remembered that in cells bounding 

 intercellular spaces the surfaces of the cell are, owing to evaporation, colder than the cell-sap, 

 which is entirely enclosed by the heat-producing layer of protoplasm. Hence a thermo-electric 

 needle lying in an intercellular space or in a pierced and collapsed cell will show a rise of 

 temperature as the warmer cell-sap exudes from the surrounding cells.] 



