We are strictly following INTERNATIONAL CODE OF BOTANICAL NOMENCLATURE in writing the names of plants.Hence we have to write the genus name first starting with capital letter and then the species name all in small letters followed by the name of the author ,who authenticated it first , and the entire name in italics.
Please note that plant nomenclature is governed by the International Code of Botanical Nomenclature (now, International Code of Nomenclature for algae, fungi, and plants). The complete scientific name of any plant should include the genus, the species, the subspecies, or the variety name, if any. The names of authors responsible for the taxonomy of a species are also cited after the Latin names. If the author’s name is too long or very familiar, it can be abbreviated, and note that in this case, L. stands for Linnaeus. In titles, full scientific name of medicinal plants including the name of authors are given.
You also asked the meaning of writing (L). When a Latin plant name is followed by an author’s name in parenthesis, and then another author’s name outside the parenthesis , the author in parenthesis is the person who first described the species and the author whose name follows the parenthesis was the first to transfer the plant to its present status. For example, in the case of water hyssop, Bacopa monnieri (L.) Pennell, Linnaeus first described it as Gratiola monnieria L. and later Pennel transferred it to the genus Bacopa