Grasses show an amazing tolerance to external conditions. For instance sheep’s fescue, which grows down to sea level in this country, has also been recorded on the highest mountains in Britain and at nearly 18,000 ft. in the Himalayas. Then again, many grasses from low-lying habitats in temperate regions adapt themselves to high altitudes in tropical countries. Others survive wide differences of climate, the classic example of adaptability being perhaps sweet vernal grass, which flourishes from sea level to above the snow line, is equally at home on sand, loam or clay, and is found in many countries of the world with vastly different climates, ranging from North Africa to Siberia.
A number of other grasses are on the other hand very specialised in their habitats. Moor mat grass is usually associated with the margins of peat moors, not because it will not grow elsewhere, but because it grows better under such conditions than any other. A number of grasses can endure strong salt water, such as marram grass (Ammophila arenaria), the salt marsh grasses (Puccinellia spp.), and sea lyme grass (Elymus arenarius), and these are confined to our coasts. Marram and sea lyme are used for stabilising wind-blown sand, while the salt marsh grasses and rice grass (Spartina townsendii) are mudbinding plants of salt marshes.
Other grasses adapt themselves well to a high water content in the soil, and “water meadows,” where periodic and controlled flooding was carried out, were in use until quite recent times. To a limited extent this is still practised in Wiltshire and Dorset. The operative phrase is “controlled flooding” for good drainage of the soil is imperative for the growth of the best types of grasses useful to the farmer. Under waterlogged conditions the deficiency of an adequate air supply to the plant roots spells failure for the ryegrasses, cocksfoot, timothy, rough stalked meadow grass, and similar productive species. Under such conditions tussock grass (Deschampsia caespitosa), rushes (Juncus spp.) and sedges (Carex spp.), which are inedible for livestock, become dominant.
The true seed of grasses is not normally seen, since the fruit-coat is very thin and firmly attached to the single seed. This type of fruit is known as a caryopsis: a wheat grain is a good example. Most grass “seeds” consist of a single grain tightly enclosed within the lemma and palea; attached to the base is generally a small portion of the axis of the spikelet. In the case of Yorkshire fog, meadow foxtail, and a number of other grasses the “seed” is, however, an entire spikelet and consists of the two glumes, and the lemma, palea and grain of one or more flowers, while in the case of the brown “seed” of timothy and most wheats the grain is shed free from its lemma and palea. The bulk of timothy seed, however, is the silver grey “seed” composed of the caryopsis complete with lemma and palea. These examples illustrate how the grass “seed”—that part of the plant which is actually sown either naturally or in agricultural practice—differs from the true seed.
Grasses are distributed in the main by the wind, for the caryopses with their enclosing glumes are light and capable of being carried long distances. Some seeds are plumed, others possess tufts of hairs which doubtless increase their buoyancy. Gales and whirlwinds are likely to exert a great influence in conveying seeds from one place to another. In Lincolnshire, for instance, a whirlwind has been known to tear up a tuft of couch grass by the roots and carry it for over twenty miles, and it has been suggested that since gale force winds are common during the period July to September, when many grasses are seeding, they must be a very important means of dispersal and probably of greater influence than normal wind-drift.
One must not overlook the influence
