ON VEGETATION. 571 



other plants. The air appears to be injurious to vegetables where it is not 

 natural ; hence arises the benefit of Mr. Forsyth's* method of completely 

 excluding the air from the wounded parts of trees, by means of which 

 their losses are often in great measure repaired, and they acquire new 

 strength and vigour. Sometimes a diminution of the magnitude of a tree 

 immediately increases its fertility ; its force being more concentrated by 

 lopping off its useless branches and leaves, it produces a larger quantity of 

 fruit, with the juices which would have been expended in their nourish- 

 ment. 



The Linnean system of vegetables is confessedly rather an artificial than 

 a natural one ; but it is extremely well adapted for practice, and its uni- 

 versal adoption has been productive of the most important improvements 

 in the science of botany. Of the 24 classes into which Linne has divided 

 the vegetable kingdom, 23 are distinguished by the forms of the flowers and 

 fruit, and the 24th by the want of a regular florescence. The first 10 are 

 named from monandria, in order, to decandria ; then follow dodecandria ; 

 icosandria, and polyandria ; the names expressing the number of the 

 stamina, or filaments, surrounding the seed vessel ; and the orders are 

 deduced in a similar manner from the number of pistils or little columns 

 immediately connected with the seed vessel ; and denominated mono- 

 gynia, digynia, and so forth, as far as polygynia. These classes differ 

 little in general with respect to their natural habits, except the twelfth, 

 icosandria, which is characterized by the attachment of the filaments to 

 the green cup, surrounding the flower, and which comprehends the most 

 common fruit trees : this class has, however, been incorporated by some 

 later botanists with the next. In the third class we find most of the 

 natural order of grasses ; the fifth, pentandria, is by far the most numerous 

 of any : the sixth contains the lilies, and many other bulbous plants. The 

 14th class, didynamia, is known by two longer and two shorter filaments ; 

 it is perfectly natural, and comprehends flowers similar in their structure 

 to the foxglove and the deadnettle. The 15th also, tetradynamia, is a class 

 of plants strongly characterized even by chemical properties ; two of the 

 filaments are here shorter than the other four : cresses, radishes, and many 

 other acrid and ammoniacal vegetables, belong to this class, as well as the 

 turnip and cabbage, which, when cultivated, become mild and nutritious. 

 The class monadelphia contains a few plants similar to the mallow ; they 

 are known by the union of the filaments at their bases into a cylinder : 

 those of the next class have generally nine united, and one separate, whence 

 the class is named diadelphia ; it contains the papilionaceous flowers, some- 

 what resembling a butterfly in their form, like the pea, and other legu- 

 minous plants, the broom, the furze, and the acacia. The 18th class, poly- 

 adelphia, has the filaments of its flowers united into several masses or 

 bundles, as the hypericum or tutsan. The next class is perfectly natural, 

 and contains the composite flowers, which have a peculiar union of the 

 summits of the filaments ; it is named syngenesia : sunflowers, daisies, and 

 artichokes, are familiar examples of the plants of this class. The 20th 

 class, gynandria, though it contains the natural family of the orchides, has 

 * On the Diseases of Forest Trees, 1791. 



