134 HOUSE, GARDEN, AND FIELD 



mechanism is not contained in the wall of the fruit, but in 

 the seed itself, which may be compared to a self-propelling 

 bullet. 



The fruit of the wood sorrel has the general arrange- 

 ment found in a field-geranium. There are five carpels 

 side by side, surrounding a central axis. The exposed 

 face of each carpel splits into halves, and these open like 

 a pair of doors to allow the seed to escape, closing again 

 immediately ; there are two or three seeds in each carpel. 



The figures (Figs. 35 and 36) show the internal structure 

 of the seed. Outside comes an elastic coat, in which the 

 motive power resides, then a number of cells, which are 

 at first packed with starch-grains, but become nearly 

 empty at the time of discharge ; then an inconspicuous 

 layer of very small cells, and a strong protective layer of 

 chestnut-red colour (the testa or seed-coat). Within the 

 seed- coat are small cells with oily contents, which serve 

 for the nourishment of the very young seedling, and lastly 

 in the centre we see the two seed-leaves of the embryo. 



During ejection everything outside the seed-coat is 

 suddenly peeled off, and it is this which hurls the seed to 

 a distance. If you were to take a golf-ball, and sew it up 

 in a stout indiarubber covering, stretching the india- 

 rubber to the utmost as you stitched it to its place, it is 

 easy to understand what would happen if the stitches 

 suddenly gave way. The indiarubber would recoil, first 

 flattening out and then bending in the opposite direction, 

 so that the concave surface would become convex, and the 

 envelope would turn itself inside out. In doing so, it 

 would strike the ball with the surface which lay in contact 

 with it, hard enough perhaps to propel it along a table. 

 In the wood sorrel seed the indiarubber layer is repre- 

 sented by the outer elastic coat, the starchy cells are mere 

 padding, now that they have given up nearly all their 

 contents, and the layer of small cells is that which suddenly 

 gives way and releases the spring. It is generally taught, 

 as for instance, by Kerner, in his Natural History of Plants, 



