HYGROSCOPIC MECHANISMS 549 



speciosa and Cynomctra cauliflora do not sink in a 3 per cent, solution of 

 brine even after being immersed for twenty-two weeks. 



Where the floating- tissue of the seed or fruit is superficial, as it is 

 in many strand-plants (Cocos nucifera, Barringtonia Catappa, Lmun- 

 itzera, Carapa, etc.), it is usually traversed by numerous mechanical 

 strands which prevent it from being easily abraded. In other cases 

 {Calophyllum inophyllum, Ximenia americana, Cycas circinalis, Excoecaria 

 Agallocha) the incoherent floating-tissue is protected by being enclosed 

 within a hard endocarp or testa. 



Kolpin Eavn has shown, that the seeds of many of our native marsh- 

 and water-plants are provided with floating-tissues of a similar nature 

 to those which have just been described. 



III. ACTIVE MOTOR-TISSUES. 



A. HYGROSCOPIC OR IMBIBITION MECHANISMS.'- 71 



The hygroscopic swelling and shrivelling of cell-walls involves 

 changes in the volume of the membranes concerned, which, under 

 suitable conditions, may produce a very appreciable amount of move- 

 ment, in the shape of a curvature or torsion of some limited portion 

 of the plant-body. It is an essential feature of every hygroscopic 

 mechanism, that the two sides of the motor apparatus which may con- 

 sist of an extensive tissue, of a single cell or even of part of a cell 

 should be antagonistic in behaviour. The principle upon which the 

 requisite antagonism usually depends may be explained as follows : 



If the lateral walls of a cylindrical cell all have the same powers 

 of imbibition, a loss or gain of water will always produce a simple con- 

 traction or extension of the cell. Curvature can only result, when two 

 opposite longitudinal strips of the cell-wall have unequal powers of 

 swelling, so that one of them undergoes a greater amount of elongation 

 than the other when water is absorbed. The same statement applies 

 mutatis mutandis in the case of a cylindrical mass of tissue. The 

 amount of curvature obviously depends in the first instance upon the 

 difference between the swelling capacities of the antagonistic sides, and 

 is unaffected by the relative thickness of the cell-walls. The energy 

 involved in the movement, on the other hand, varies directly as the 

 thickness of the active membranes. As a matter of fact, hygroscopic 

 curvatures of plant-organs always take place in opposition to more or 

 less powerful resistances ; hygroscopic motor-cells are, therefore, usually 

 more or less thick-walled. 



The change in the volume of a cell-wall which accompanies a change 

 of water-content, depends not only upon the quantity of water removed 

 or introduced, but also upon the molecular or micellar structure of 



