MOVEMENT 



363 



seed in the earth. The bursting of seed pods when they dry out 

 is due to similar torsions Bel up by unequal water losses. The 

 elaters of the horsetails (Equisetum) and the teeth of the peri- 

 stome of the mosses function in the same way, thus helping to 

 scatter 1 he spores. \^ry rapid hygroscopic movements of a similar 

 nature occur in the bracts of Ammcbium, Rhodanihe, and other 

 Composite (Uphof, 1924). 



In the cases just described, the cell wall loses water, causing 

 the difference in tension and the resulting motion or curvature. 

 In the sporangia of the ferns the mechanism is somewhat differ- 

 ent. When the cells of the annulus dry out, the walls remain 



Fig. 27. — Opening of fern sporangium due to imbibition, a, sporangium 

 starting to open; b, with annulus bent hack scattering the spores; c, sprung 



back to the normal position but empty. 



saturated with water as the water moves out from the lumina to 

 replace that lost by evaporation. The watery contents adhere 

 to the wall, but at the same time the cohesion of the water mole- 

 cules for each other is also very great (Chap. XXI). As the water 

 content decreases, the cell wall thus becomes deformed and the 

 thin outer walls of the annulus cells are pushed in toward the 

 opposite sides, which are much thicker (Fig. 27). The entire 

 annulus is thus shortened, with the result that it ruptures al the 

 side where the cells are weakest. The surface tension forces draw 

 the annulus slowly backward, when the water menisci in the cells 

 suddenly break, with the result, thai the top is thrown back into 

 place, violently catapulting the spores (Conard, 1908). In some 

 ferns, this may later be repeated. If water is added, the spo- 



