The Dispersal Of Seeds Of Waste-Ground Plants

The diversity of habitats of waste-ground plants is perhaps correlated with the different modes of dispersal of the seeds. As a whole the habitats are not open, and require special means for the dispersal of the seeds. The prevalence of annuals demands the production of a large number of seeds, and a large proportion of the plants, as the Docks and Goosefoots, produce a considerable number of flowers. The gregariousness of many of the plants also within limited areas has an important bearing upon the mode of dispersal.

The agency of man in distributing such aliens independently of the mode of dispersal to a large extent determines their distribution. Railways and canals are important agents in dispersal; but principally there is the carrying on of agriculture and the carriage of crops from one spot to another, from the field to the stackyard. The manuring of ground also plays an extensive part in the spreading of aliens or other types of waste-ground plants.

Some plants depend upon their own mode of dispersal, as Greater Celandine (which is also dispersed by ants), Shepherd's Purse, Mallow, Stork's Bill (in which the seed are dispersed by an elastic movement, and in which the long awn is hygroscopic), Melilot, Viper's Bugloss, Henbane (also wind-dispersed), Mullein, Creeping and Common Toadflax, Purple Dead Nettle, White Dead Nettle, Fat Hen (also wind-dispersed), Good King Henry (also wind-dispersed). In many cases the seeds of fruits are small; and in other cases, as in numerous Composites, provided with hairs and dispersed by wind, as Mouse-ear Chickweed, Goutweed (flattened fruits), Stinking Mayweed (winged achenes), Tansy (achene membranous), Groundsel, Musk Thistle, Spear Thistle, Milk Thistle, Chicory, Hawksbeard, Knotgrass, Dock, Wall Barley.

The burs of Burdock are hooked and catch in the wool of sheep, and on recoiling are shot to a distance away. Hound's Tongue, Bittersweet, and Belladonna, are also dispersed by animal agency.

The Soil Of Waste Places

The artificial character of waste ground to some extent causes the plants that colonize it to become independent of soil conditions. The variety of habitat ensures the variety in the soil as a whole. The proximity of some types of habitat to farmyards, and manure heaps in particular, and the dominance of luxuriant, sturdy weeds upon them, may be due to the ousting of weaklings, or to the antipathy of other plants for richness of soil with abundance of nitrogenous materials.

Quarries cut out of solid rock are excavated in various rocks giving diverse types of soil, and the alien plants grow upon the waste materials, which have little effect upon them apparently.

By far the greater number of waste-ground plants described in this chapter are usually found upon sandy or loamy soils. A few require some proportion of humus whilst tolerating a sandy soil. These are, in general, hedgerow plants, as Greater Celandine. Some require clay or humus, as Goutweed, Spear Thistle (clay or sand), Purple Dead Nettle (clay), White Dead Nettle (clay or sand), Dairy Maid's Dock (clay or humus).

Stork's Bill and Hound's Tongue, whilst growing on sandy soil, are frequently Halo-phytes, growing on sandy coasts in many districts. Viper's Bugloss, Creeping and Common Toadflax, grow also upon chalk or limestone, but on waste ground frequently upon sand.