Introduction to seed plants
Seed plants are the familiar green components of wild, modified, and managed landscapes. They provide many of the things that are important to humans and other animal species: timber and paper, shelter, food, drugs, clothing, flowers, and, along with other photosynthetic organisms, they sustain life by converting sunlight into chemical energy and providing atmospheric oxygen.
Cortaderia richardii. Image - M Dawson
Within the seed plants, two groups are traditionally recognised:
- Gymnosperms. The name gymnosperm means ‘naked seed’, a reference to the absence of an ovary. The 733 extant species of gymnosperms are classified into four main orders (Judd et al. 1999), the largest of which is the Coniferales. All New Zealand´s 20 native gymnosperms are conifers, classified in three families (Hart 1987; Kelch 1997, 1998).
- Angiosperms.The angiosperms, or flowering plants, are characterised by living companion cells in their inner bark that help organise the flow of nutrients; by a special triploid nutritive tissue, the endosperm, in their seeds; by their seeds enclosed in special structures, the carpels, hence the name angiosperm – ‘enclosed seed’; and by their most obvious feature, the flower, which is variously modified and provides the important characters for classification of the group. The angiosperms comprise about 234 000 living species (Thorne 1992).
New Zealand has about 5000 indigenous and exotic seed plant species, with an estimated 25 000 species in cultivation. Twenty species of conifers and over 2000 species of flowering plants are native in New Zealand. All the native conifers and over 80% of the indigenous flowering plants grow nowhere else on Earth.