180 FLOWERS OF WASTE PLACES, ETC. 



expelled from the pod by an elastic movement, and are drawn into the 

 ground by a similar hygroscopic arrangement. The carpels do not open 

 but are contracted with the seeds still enclosed, the awn remaining 

 attached. The layers of the cell-wall consist of lamellae of different 

 densities and refractive indices. In the one the cells are elongate and 

 woody in concentric series, being light and dark alternately. These 

 last are the edges of the lamellae. The parallel lamellae in two series 

 are inclined to the axis at different angles, or wind spirally in opposite 

 directions around the lamellae. The expansion of the cell-wall tissue 

 during imbibition is caused by the swelling of the striae of less density, 

 and the imbibition of the water in all probability sets up spiral tension, 

 producing a twisting motion. Further single cells roll up as shown 

 by Francis Darwin. Or the twisting of the awn may be clue to the 

 difference between the contraction of the woody fibres and the com- 

 paratively soft parenchyma or thin- walled cellular tissue, in which the 

 cells are not much longer than broad. The more complete lignifica- 

 tion of the outer cells, which contract more than the inner, may be 

 the reason, the spiral twisting being due to the curving of the woody 

 bars with the hollow side upwards. Moistness regulates the amount 

 of the twisting of the awn, which twists and untwists with variation in 

 atmospheric humidity, being thus hygroscopic. The fixing of the awn 

 during the process of untwisting causes the seed to be driven into the 

 ground. The seed vessel is so sensitive the arista or awn curls up 

 under the influence of the heat or moisture of the hand. The elastic 

 movement of the seed to promote dispersal is one of the most inter- 

 esting examples of sensitiveness. 



Stork's Bill is almost entirely a sand plant, growing almost always 

 on sand soil. By the sea-coast it is a halophyte, living on a saline soil. 



The Brown Argus (Lyccena astrarclie] feeds upon it, also Geotoimis 

 piLiictulatus and H eterogaster 2irtic<z, Hemiptera Heteroptera. 



Erodium is from the Greek E radios, a heron, in allusion to the 

 shape of the fruit. Cicutarium is from cicuta, hemlock, because the 

 foliage resembles that of hemlock. The plant is called Wild Musk, 

 Pink or Powk Needle, Stickpile. 



ESSENTIAL SPECIFIC CHARACTERS: 



69. Erodium ciciitarium, L'Herit. Stem prostrate, spreading, 

 downy, leaves pinnate, leaflets sessile, pinnatifid, stems many-flowered, 

 flowers lilac, in umbels, stipules lanceolate, stamens dilated. 



