149 DISPERSAL OF SEEDS. 
than it would be if all contained seed, and probably enables the wind 
to carry it a long distance. I have made three experiments in 
weighing small seeds. PaPayA DUBIUM about 460 per grain; 
ANTIRRHINUM 440-450 per grain; ORoBANCHE MINUS (lesser Broom- 
rape) about 12,500 per grain, this latter does, no doubt, get blown 
about among dust. 
The next section is that in which the plant has some mechanical 
contrivance for throwing the seed, and, in some cases, for burying 
them. In other cases a kind of miniature trap-door to let the seeds 
out when they are ripe and when the weather is favourable. 
Take our common Land Cress (CARDAMINE HirsUTA). It has 
small white flowers followed by long narrow upright pods, consisting 
of two valves, and a thin membraneous septum between them, to 
which the seeds are lightly attached by very short stalks. When 
the pods are ripe the valves are in a state of tension. So that the 
least touch, or a puff of wind, causing one plant to touch another, 
the two outer valves suddenly roll themselves up, often with force 
enough to detach themselves from the plant, throwing the seeds with 
considerable force to a distance of a yard or two. 
The common Dog Violets (Vioua CANINA or V. SYLYATICA) and 
the numerous sub-species, etc., are most interesting in the way 
they discharge their seeds. 
The flower stands boldly up above the leaves, but as soon as it 
fades the flower stalk turns down, and the young capsule is found 
hanging down among the leaves. As soon as it is ripe the stalk 
again becomes erect, and the capsule splits into three equal boat- 
shaped valves, each containing three to five seeds inonerow. These 
seeds are very smooth, and pear shaped, with the pointed end towards 
the bottom of the valve. As the valve dries its sides contract 
inwards, thus exerting pressure on the seeds, tending to squeeze 
them out. They resist for some time until at length the squeezing 
