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383 



diate vicinity of the separation-layer, as compared with the same 

 kind of cells half way down the pedicel, shown in figure 5. On 

 the distal side of the separation-layer, the walls of the pith-cells 

 remain thin and unchanged (fig. 7), except for a few rows in the 

 immediate vicinity of the layer. The slender, pointed cells of this 

 tissue are interpreted to mean that they have grown under pressure, 

 which caused them to glide between neighboring cells, as oppor- 

 tunity offered. The pressure 

 involved was produced by the 

 elongation of the thick-walled 

 cells. By referring to figure 5 

 it will be seen that the rigid, 

 thick-walled cells of the pith do 

 not glide between each other. 

 The mechanical tissues develop 

 regularly and form the cylinder 

 described above. 



The fibro-vascular system 

 may be compared to a thin- 

 walled, inelastic cylinder (fig. 

 3, D), inside of which the elon- 

 gating pith-cells (fig. 3, E) act 



lik 



e a piston moving against 



the soft, spongy, pith-cells (fig. 

 3, I) in the closed end of the 

 rigid cylinder. The cushion of 

 thin-walled cells is thus analo- 

 gous to a compressed spring 

 (one end of which presses 

 against the achene and the 



Fig. 5. Longitudinal section of pith 

 taken about midway between the base 

 and apex of the pedicel. 



other against the separation-layer). So long as the structure is un- 

 disturbed, the fibro-vascular cylinder restrains the elongating force 

 of the pith column, but when the rigid style is struck by sufficient 

 force, the strained walls of the cylinder break at the separation- 

 layer and the compressed cushion of cells, suddenly expanding 

 like a released spring, throws the achene off with considerable 

 force. The invariable breaking- of the fibrovascutar bundles at the 

 separation-layer is not hard to explain. An examination of a 



