SECT. II PHYSIOLOGY 297 



on imbibition. The COHESION mechanisms (^'^) were previously con- 

 founded with the latter, from which they diifer in that, even during 

 the movement, the cell-walls remain saturated with water. It is the 

 lumen of the cell which diminishes in size Avhen the loss of water, on 

 which the movement depends, occurs. The cohesive force of the 

 diminishing amount of water tends to pull the thinner walls of the 

 cell inwards after it (L^'ig- 225), and thus to approximate other walls, 

 which are strongly thickened and exhibit a definite arrangement. 

 In this way an energetic shortening of the specialised tissue is brought 

 about which leads to alteration of form or to the dehiscence of spaces 

 enclosed by the tissue. The walls of anthers and of the sporangia of 

 the higher cryptogams afford examples of such a method of dehiscence. 

 The movements of the pappus of most, and the involucre of some, 

 Compositae, are eftected in the same way. 



Mechanisms dependent on imbibition and on cohesion may co- 

 operate in the movements of some plants. 



III. Movements in the Actively Living" Plant 



These movements are to some extent dependent on the factors 

 which we have recognised as general conditions of life. They are 

 most active at an optimal temperature and humidity and sometimes 

 also intensity of light. On overstepping the minimum or the 

 maximum for these factors a loss of motility or a condition of rigor 

 results. Thus we speak of cold-, heat-, darkness-, or drought-rigor, 

 etc. This condition can be removed by a return of the favourable 

 conditions, but if it lasts long enough will ultimately lead to death. 

 In some cases it is sufficient that these general conditions of life should 

 be present, but in others the movement only results on the application 

 of a special stimulus. We have, therefore, autonomic movements and 

 induced or stimulus movements to study. 



A. Autonomic Movements 



The creeping movements of plasmodia and amoebae, the swimming 

 movements of swarm-spores and gametes and the protoplasmic stream- 

 ing in cells are frequently, though not always, autonomic. That is to 

 say we cannot refer the start of the movement to one or several 

 external factors. When these movements are purely autonomic they 

 usually tend to be without definite direction. The plasmodia move 

 without object, the swimming movements and the circulatory move- 

 ments alternate continually, and only the current of the rotating 

 protoplasm exhibits a definite direction. 



Thus also the growth of the shoot or root in a straight line (with 

 the characteristic grand period of growth, dependent as has been 

 shown wholly on internal causes) is an autonomic movement. A 



