422 TRANSFORMATION OF ENERGY 



in a straight line in water or air, but they reach high values when, for example, 

 a root attempts to force its way into the ground or encounters stones in its 

 passage through the soil. A considerable amount of work must often be accom- 

 plished when curvatures take place, and the amount is all the greater the nearer 

 to the base of the organ the zone of curvature lies, for the weight of the passive 

 erect portion of the plant is so much the greater. 



It would appear that the energy for carrying out these variation move- 

 ments can be obtained only from osmotic pressure, but since, as we previously 

 saw, far greater forces are developed in surface growth of the cell-wall by 

 excretion-energy than by osmotic pressure, we may assume that excretion- 

 energy also plays an important part in these external activities of the growing 

 plant. According to PFEFFER'S researches, although these external activities 

 are to be referred exclusively to turgor pressure, the plant is able, however, to 

 employ the whole of that pressure in the overcoming of resistance. 



Without going into detail, we may merely note that PFEFFER imbedded 

 the part of the plant under investigation in plaster of Paris, forming a general 

 resistant layer, and, with the aid of appropriate apparatus, was able to measure 

 the pressure which the plant exerted to overcome the resistance thus given to 

 its expansion. External pressures of this kind often reached as much as twelve 

 atmospheres. 



So long as the cell encounters no external obstacle to its expansion the 

 whole of the osmotic energy is devoted to the stretching of the cell-wall, but 

 after enclosure in plaster of Paris the wall is extended by growth, and, as the 

 extension progresses, the osmotic pressure is directed against the obstacle. 

 When extension is complete the whole internal pressure may in the long run be 

 devoted to the performance of external work, and in many cases, as the resist- 

 ance increases, not only does the extension of the cell- wall increase up to its 

 limits but the osmotic pressure also rises above the normal. 



The first series of movements we have to study are those which are illus- 

 trated by ripe fruits and spores, and which may be termed ejaculatory move- 

 ments. All of these are characterized by their suddenness, and that sudden- 

 ness is rendered possible only by tensions induced between tissues, cells, or parts 

 of a cell, and which are equalized in a moment. A phenomenon such as this 

 we have already recognized as taking place in hygroscopic movements, where, 

 in addition to the slow oscillation due to gradual absorption and evaporation of 

 water, irregular movements also occur. At the moment when the dehiscence of 

 a capsule is effected by a sudden adjustment of tensions, not infrequently parts 

 of the fruit wall or the seeds are thereby thrown off. The ejaculatory movements 

 we have yet to study may be compared, from a biological point of view, with 

 these movements in dry fruits. So far as the mechanism is concerned they are 

 closely related to them, but the difference lies in this, that the tensions previously 

 mentioned are conditioned by the swelling of the membrane or cohesion of imbi- 

 bition water, while those we have now to speak of are due to osmotic pressure. 



Let us commence with cases where the tensions are those which occur in 

 single cells, as, for example, in the spore cases (asci) of the Ascomycetes (DE 

 BARY, 1884), and let us select Ascobolus as our first example. The asci are 

 elongated cells, each containing eight young cells or spores in addition to the 

 normal cell constituents, viz. a peripheral protoplasmic layer much reduced in 

 amount and an osmotically active cell-sap. The asci are aggregated in 

 thousands into a single layer (hymenium), and are mixed with narrow sterile cells 

 (paraphyses). When the ascus becomes ripe a marked increase in the osmotic 

 pressure occurs, along with a subsequent increase in volume, which may easily 

 result in the doubling of the original length and diameter of the ascus. If the 

 ascus is cut off or plasmolysed it regains its original size, so that the increase in size 

 is not due to growth. At a certain moment in the natural course of develop- 



