PHYSIOLOGY 



271 



The variation in dampness of the air causes the pappus hairs of the 

 Cynareae (Compositae) to expand in dry and fold together in damp weather. 

 The opening or closing of the moss sporogonium is, in like manner, due to the 

 hygroscopic movements of the teeth of the peristome surrounding the mouth 

 of the capsule. In the case of the Equisetaceae the outer walls of the spores 

 themselves (the perinium) take the form of four arms, which, like elaters, are cap- 

 able of active movements, by means of which numbers of spores become massed 

 together before germinating, and the isola- 

 tion of the dioecious prothallia prevented 

 (Fig. 420E). 



In order to call forth imbibition move- 

 ments the actual presence of liquid water 

 is not necessary ; for the cell walls have 

 the power of absorbing moisture from the 

 air. They are hygroscopic, and for this 

 reason the ensuing movements are also 

 often termed hygroscopic movements. 



STEINBRINCK and KAMERLING 

 have distinguished the mechanisms 

 which depend on the cohesive power 

 of water from those depending on 

 imbibition. The cohesion mechan- 

 isms were previously confounded 

 with the latter, from which they 

 differ in that, even during the move- 

 ment, the cell walls remain saturated 

 with water. It is the lumen of the 

 cell which diminishes in size when 

 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, 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 elaters of 

 Liverworts and Myxomycetes, which serve to distribute the spores, 

 as well as of the pappus of most, and the involucre of some, Com- 

 positae, are effected in the same way ( 78 ). 



Mechanisms dependent on imbibition and on cohesion may co- 

 operate in the movements of some plants. 



2 Pai frnitrf J5rKw jrf MW. 



A, in the dry condition, coiled; B, moist 



., ,., 1(11 , iltl .;,. 



