There is always someone recounting the number of species on Earth and the numbers usually differ! Accepting the limits of the guesstimates involved perhaps this list will give some idea of the scale.

Statistic on Number of Species (2011 estimates)

• Total number of species 8,700,000 eight point seven million. 86% still undiscovered.
• Land based species 6.5 million sea based 2.2 million – seems low for the sea to me!
• Number of animals 7,770,000 three quarters of which are Arthropods (insects and crustaceans)
• Total of all land plants including trees, mosses, ferns, conifers, flowering plants, and so on = 298,000. (I bet I have that many weeds in my garden.)
• Fungi 611,000, protazoa and chromista 64,000 (and the dollar question is what are they).

ref Dr Camilo Mora Census of Marine Life

Biological Classification Table

Creative commons share alike license from wikipedia

Plant Classification

Species
The rank of species is an important botanical classification. Plants within a species can be very different for example consider the types of Apple or Rose.
‘A species is a group of plants with similar DNA that are capable of interbreeding and producing fertile offspring.’
The creation of genetic variation in a plant species and the selection from within that variation of plants with desirable and inheritable traits, forms the basis of one or more plant varieties.

Variety
This has two similar meanings. It is a legal term for a cultivated plant protected under UPOV
Some horticulturists use “variety” imprecisely; for example, viticulturists almost always refer to grape cultivars as “grape varieties”.

Cultivar or Variety
Cultivars are produced by careful breeding, propogation and selection for flower colour and form on plants such as roses, rhododendrons and many farm & food crops.
A cultivar name consists of a botanical name of a genus, species and taxon followed by a cultivar epithet. Cultivar names are always written in Roman letters enclosed in single quotes, like Hosta ‘August Moon’ it should not be italicized.
Rosa ‘Peace’ “Peace” is a trade designation or “selling name” for the cultivar R. ‘Madame A. Meilland’ and should therefore be printed in a different typeface from the rest of the name, without quote marks, for example: Rosa Peace.

Hybrids.
Hybrid consists of crosses between different cultivars within a single species. This meaning is often used in plant breeding, where hybrids are commonly produced and selected because they have desirable characteristics not found or inconsistently present in the parent. Occasionally plants from two different species can be hybridised or ‘crossed’. They are usually given a collective latin name or English name Quercus x hispanica or Rhododendron Lady Chamberlain x Gleam.

Common Names
Common names may be local or descriptive but are too inconsistent to be part of a plants formal name for identification purposes.
An example may be Quercus coccinea “Scarlet Oak”(common name)

Plant Names and Protections

The International Plant names index has records from three sources: the Index Kewensis 1million plus, the Gray Card Index 350,000 and the Australian Plant Names Index 63,000. Whilst needing some deduplication it ‘represents the most comprehensive listing of plant names available today’.

The Plant Varieties Act 1997 establishes rights, known as “plant breeders’ rights”, in plant genera and species that qualify for protection under that Act. British Plant Breeders’ Rights protect creators of new varieties against ;

• production or reproduction (multiplication),
• conditioning for the purpose of propagation,
• offering for sale, selling or other marketing, exporting, importing, stocking for any of the above purposes or
• any other act prescribed for the purposes of that provision without his authority.

The International Union for the Protection of New Varieties of Plants or UPOV grants plant breeders’ rights to new varieties that meet four criteria ;

1. The new plant must be novel, which means that it must not have been previously marketed in the country where rights are applied for.
2. The new plant must be distinct from other available varieties.
3. The plants must display homogeneity.
4. The trait or traits unique to the new variety must be stable so that the plant remains true to type after repeated cycles of propagation.