COHESION MECHANISMS 



559 



to the writings of Leclerc du Sablon, Goebel, Steinbrinck, and Ur- 



sprung ; the last-mentioned author states that hygroscopic action plays 

 a part in certain cases. 



According to Kanierling, the elaters of Liverworts also afford an 

 excellent illustration of a cohesion mechanism. Typical elaters occur 

 in most Jungermanniales and Marchantiales. They are long, spindle- 

 shaped structures, which, after the dehiscence of the sporangium, 

 execute jerky movements, and so help to break up the spore-mass, and 

 to disseminate the individual spores. Each elater lias a thin wall 

 provided with two parallel spiral thickening fibres. When water is 

 withdrawn from the cavity of the elater, the' thin strips between the 

 spiral fibres are drawn inwards, owing to the internal cohesion of the 



A. Diagram of a leaf of Festuca glauca, in T.S. ; a-a, leaf expanded ; 6-?), leaf folded 

 up. Sub-epidermal fibrous tissue of the lower side shaded. B. T.S. across one of the 

 furrows on the upper side. Note the large epidermal cells at the bottom of the 

 furrow. After Tschirch. 



liquid ; the radius of curvature of the fibres consequently diminishes, 

 and the number of spiral turns is increased. In other words, the 

 elaters undergo torsion, and the thickening fibres become wound up 

 like the mainspring of a watch. When the tension in the membrane 

 becomes greater than the internal cohesion of the water, the spiral 

 fibres return with a sudden jerk to their original position. 



Some of the movements of vegetative organs also depend upon 

 cohesion. The leaves of many xerophilous Grasses (especially steppe- 

 inhabiting species) become folded, or curl up, when they are insuffi- 

 ciently supplied with water, in order to avoid excessive transpiration 

 (Fig. 228). Such curling or rolling Grass-leaves are furnished with a 

 number of longitudinal grooves on the upper side. The epidermal 

 cells which are situated at the bottom of these furrows, are dis- 

 tinguished by their large size (especially as regards their vertical 

 diameter), and by the thin and flexible character of their walls. The 

 abaxial half of the leaf contains sub-epidermal fibrous ribbons or 

 strands (Fig. 228 a). Opinions differ with regard to the precise 



