CHAP, IV. TLAN or DESCIlirXION AND HISTORY. 225 



in certain soils, &c. ; will be noticed as far as they are known : for, 

 though none of these peculiarities can be continued by propagation, 

 some of tliem may be produced by culture. 



f. Impulsion, or Rate of Groxcth. The number of feet, or inches, made 

 by shoots of one season's growth, in trees of different ages, will be 

 given ; and the height which the species generally attains in ten, in 

 twenty, in thirty years, and when full grown, in the environs of the 

 metropolis, will be stated as f\ir as has been ascertained. 



g. Metamorphoses and Degeneracies. The doctrine, that all the parts of 

 a plant may be reduced to two (viz. an axis, and a leaf revolving round 

 it), seems to have been hinted at by Linnwus, but was first brought 

 forward in a conspicuous manner by the poet Giithe. (See De 

 Cand., Theorie E'lementaire,\). 105. and Physiohigie Vegetale, p. 771., 

 and Gothe's Versuch ilher die Metamorphose der I'finvzen, 1 831 .) The 

 doctrine is now generally adopted by botnnists ; and double flowers 

 and various other monstrosities and transformations are referred to 

 this head. 



b. The Anatomical Structure of species will be noticed when it Is, in any 

 respect, remarkable ; as, when it influences nuiterially the texture or 

 veining of the timbei', or the susceptibility of the plant to be united 

 to others by grafting, its fitness for resisting wind, &c. 



i. Phijsiology. Anything remarkable in the functions of any species will 

 be stated ; together with its bearings on propagation, culture, or 

 duration : such as the kind of sap, whether milky or watery, sugary 

 or alkaline, &c. 



k. The Affinities of Species constitute an important part of their study, 

 with a view to their propagation and culture. Some species may be 

 grafted, not only on every other species of the same genus, as in the 

 case of Cratffi^gus, but on every species belonging to the snme tribe, 

 such as Prunus : other species, which will not unite by grafting to all 

 the species of their own genus, such as Py-rus communis, which will 

 not unite toPyrusil/alus, will yet unite to Cratje^gus and Morbus. In 

 general, plants which have miky sap will not unitewith such as have 

 watery sap, and, indeed, will not unite with other plants at all. Hence, 

 A'cQv platanoldes, according to De CandoUe {Physiologie Vegetale, 

 vol. ii. p. 794.), cannot be grafted on any other species of the genus. 



1. Resemblances. Some trees and shrubs bear a resemblance to one an- 

 other without having any affinity, either organic or physiological; for 

 example, the different species of C'arj'a, iihiis, and Aildntus ; or the 

 common laurel and the Alagnoh'a grandiflora. These resemblances 

 it will be useful to notice, with a view to ornamental plantations. 



m. Contemporary Foliation, Flowering, and Defoliation. For the same 



purpose as that of indicating resemblances, it will be desirable to note 



trees and shrubs which come into flower at the same time ; or which 



either come into leaf, or shed their leaves contemporaneously. 



6. Casualties. Trees and shrubs are liable to be preyed on by insects, to be 



injured by vermin and parasitical plants, attacked by diseases, and broken 



down or destroyed by accidents, 



A. Insects and Vermin. The particular species of these which are either 

 peculiar to certain species of trees and shrubs, or liable to attack them, 

 will be described, and occasionally figured ; and the means of protection 

 from their ravages, when known, or of alleviation, or of cure, will be 

 pointed out. 



B. Parasitical Plants. Trees and shrubs are liable to be injured by the 

 growth of lichens, mosses, and other parasites, on their leaves, bark, and 

 wood ; and by Piiugi on their bark and leaves : among the latter class 

 is the mildew. 



C. Diseases. The diseases to which trees and shrubs are liable, exclusive 

 of the injury done to them by insects and vermin, and by parasitical 



82 



