An arboretum (from Latin arboretum, orchard) is a collection of trees.
The Conifers (from the Latin coniferus, bearing cones) are an order within the Gymnosperms. Most common species in the northern hemisphere are firs (Abies), spruces (Picea), pines (Pinus), hemlocks (Tsuga), larches, (Larix), cedars (Cedrus), cypresses (Cupressus), junipers (Juniperus), Sequoias and so on. See Taxonomy.
The cotyledons are the first leaves of a plant after germination. They are usually quite different from the leaves that will grow later and can be typical for a plant or a group of plants. Among conifers, their number is highly variable between the genera and even between the species (example : Pinus), from 2 (some Cupressus, Araucaria [sometimes 3], Cephalotaxus, Juniperus) to 24 cotyledons (Pinus maximartinezii - highest number of all plants).
Plants are said dioecious when they have male and female cones or flowers on different individuals. Examples : Cephalotaxus, Juniperus, Taxus.
The germination is epigeal when the cotyledons will raise above the soil surface. Examples : most plants, Abies, Cupressus, Juniperus, Picea, Pinus, Taxus.
The gymnosperms (from Greek, 'naked seeds') are a group of flower plants with naked ovules and then naked seeds, non enclosed in an ovary. See Taxonomy.
The germination is hypogeal when the cotyledons will remain under the soil surface. Usually the cotyledons will stay inside the seedcoat. Examples : Araucaria, Cephalotaxus - partially, Ginkgo, Keteleeria, Torreya.
Plants are said monoecious when they have male and female cones or flowers on the same individual. Examples : Abies, Cupressus, Picea, Pinus.
A pinetum (from Latin pinetum, forest of pines) is a collection of conifers.
A plant is said to regenerate itself when new seedlings will appear naturally and develop to maturity without any human intervention. This concept is used for cultivated plants, especially for introduced species, or when a plant is endangered because the environmental conditions (climate, browsing by animals for instance) have changed not allowing anymore the emergence of new plants. One example is Cupressus dupreziana growing in the Sahara Desert (Tassili) : there are only a few old trees left and the produced seeds will not find anymore a way to establish themselves under the desert environment.
Taxonomy is the science of the classification of all living beings (past and present - from bacteria to mushrooms to plants and to animals ). This classification is traditionally based on the phenotypic characters of the different living beings. Nowadays DNA analysis is more and more used.