Evidence of grasses begins to appear in the fossil record of some 65 million years ago. Their pollen grains, which have a distinctive durable coat, feature in soils laid down in that period. By 34 million years ago, the grassland-vegetation type had become widespread and animals had developed the specialised grinding teeth capable of coping with tough silica-rich grass leaves.

Using evidence from leaf and flower structure, together with molecular data and information on their photosynthetic pathways, botanists can establish which are the most primitive grasses and how grasses have evolved.

Blowing in the wind

Many members of this sub-family are perennials with tall woody stems recognisable as bamboo ‘canes’. Commonly found in forests in warm temperate or tropical regions, they have the C3 photosynthetic pathway and usually flower gregariously at long intervals of many years. Other species are herbaceous plants that flower annually; they are characteristically found in tropical forest shade. Although the spikelet structure of bamboos is simple, the spikelets may be grouped together in dense and complex inflorescences. Well-known members of the Bambusoideae are the bamboos (eg Phyllostachys, Arundinaria ).

In evolutionary terms, woody bamboos offer an intriguing mixture of primitive and advanced characters. Like many other monocotyledon families, they have floral parts in threes. Other grasses have more advanced flowers, with the number of floral parts mostly reduced to two. In contrast, the development of woody, long-lived stems appears to be a unique advancement within the grass family. Likewise the complex inflorescence suggests considerable evolutionary progression.

Reed sub-family (Arundinoideae)

This sub-family of tufted and tussock grasses and large reeds is generally found in the southern hemisphere’s temperate and sub-tropical regions, often in savanna grassland or other open dry habitats. Among its members are reeds (Arundo, Phragmites), pampas grass (Cortaderia) and purple moor grass (Molinia).

With their unspecialised floral structure and the C3 photosynthetic pathway, these grasses possibly appeared early in the grass family’s evolutionary history. They probably originated in the tropics but retreated southwards in the face of competition from grasses with the more efficient C4 photosynthetic pathway.

Aristida sub-family (Aristidoideae)
These grasses are mostly found in the dry tropics and sub-tropics. Their distinctive seeds have three awns (stiff bristles). Their C4 photosynthetic pathway is an adaptation to high light and low water availability.

Rice sub-family (Ehrhartoideae)
Members of this small sub-family occur in both dry and wet locations. Their floral structure is simple and they have the C3 photosynthetic pathway. They include rice (Oryza), the most important cereal crop of the tropics.