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THE PLANT WORLD. 
tering of the seeds is brought about by a mechanical contrivance 
within the fruit. About the time the seed is ripe, a layer of long 
thin-walled cells near the apex of the berry become filled with 
water. The increase in turgidity of this, the expulsory layer, 
causes a strain on the layer of cells at the point where the berry is 
attached to its stem. This scission layer, as it is called, is rup- 
