The dead remains of these sociable grasses are an important factor in protecting the soil against drought and in facilitating humification, as well as in covering up plants during long winters or dry seasons, keeping the ground warmer and moister, and generally lessening the effect of extremes.
Many Graminaceæ are pronounced xerophytes, the epidermis often being developed as a water-storing tissue, while the erect leaves roll themselves in intense light, the stomata being situated accordingly. The halophytic strand-plants Psamma arenaria, Elymus arenarius, Agropyrum junceum, and other Dune-species, as well as species of Aira, Festuca, Anthoxanthum, Stipa, Lygeum, Aristida, &c. are examples. The heath-grasses—e.g. Festuca ovina, Nardus stricta, Molinia cærulea—also come under this category.
Many of the strand-plants (halophytes) Agropyrum, Psamma, Elymus, are covered with waxy bloom, and have long rhizomes which bind the sand and form new soil, a property largely taken advantage of in certain forest operations.
Other grasses, particularly annual species, show their adaptation to xerophytic habits by forming bulbous store-houses at the base of the culms—e.g. Phleum arenarium.
Some Graminaceæ are hydrophytes, such as Arundo, Glyceria, &c., with large intercellular spaces in their tissues; while many species—e.g. Aira cæspitosa, Agrostis canina, Molinia cærulea—grow on wet moor-lands, forming perennial tufts, with or without creeping rhizomes.
The mesophyte grasses are especially characteristic of what may be termed carpets—a lawn is a good example on a small scale, though of course we must remember that here the struggle for existence has been artificially interfered with more or less. Such carpets consist of the densely interwoven rootlets and rhizomes forming sod, and contain much humus from the accumulated débris of former years. These grass-carpets may be composed of nearly pure growths of a few species, or of very many different grasses and other herbage. They are common in Arctic regions, on Alps, and in temperate climates generally, where we know them as meadows, hay-fields, pasture and lawns.
The Bamboos in the wider sense have a physiognomy of their own, e.g. in India, and may drive out most other plants and form dense undergrowths or jungle of interlaced stems and leaves and thorny shoots. Similar growths occur on the Andes and elsewhere in South America. In some parts of India and tropical Asia the taller bamboos form aggregates comparable to dense forests, and such forests are common on the banks of several large tropical rivers. Most of these Bamboos are xerophytes. Bamboos are neither confined to the tropics, nor to warmer regions, however, for species are known from distinctly cool regions—e.g. South America—or even from near the snow line—e.g. Chili, the Himalayas, Japan, &c., and the number of species known as hardy is increasing annually, as is evident on examining our larger English gardens.
The permanence and character of extensive grasslands, especially prairies, savannahs, and steppes, are much affected by the periodical firing they are exposed to in the dry season, and large tracts of country in various parts of the world would doubtless bear forests or other vegetation if not thus fired, while in other cases the herbage would be differently constituted were firing discontinued.
The following chapter embodies an attempt to classify our British grasses solely for purposes of identification when not in flower. It is not claimed that the arrangement is the best possible, nor that it is complete, and I need hardly say that corrections will be gratefully received.