CHAP. IV. PLAN OF DESCRIPTION AND HISTORY. 295 
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 
them may be produced by culture. 
f. Impulsion, or Rate of Growth. 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 far 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 Linnzus, but was first brought 
forward in a conspicuous manner by the poet Géthe. (See De 
Cand., Théorie E’lémentaire, p. 105. and Physiologie Végétale, p.771., 
and Gothe’s Versuch tibey die Metamorphose der Pflanzen, 1831.) The 
doctrine is now generally adopted by botanists; and double flowers, 
and various other monstrosities and transformations, are referred to 
this head. 
h. The Anatomical Structure of species will be noticed when it is, in any 
respect, remarkable ; as, when it influences materially the texture or 
veining of the timber, or the susceptibility of the plant to be united 
to others by grafting, its fitness for resisting wind, &c. 
i. Physiology. 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 Cratz‘gus, but on every species belonging to the same tribe, 
such as Prinus: other species, which will not unite by grafting to all 
the species of their own genus, such as Pyrus communis, which will 
not unite to Pyrus Malus, will yet unite to Cratz‘gus and Sérbus. In 
general, plants which have milky sap will not unite with such as have 
watery sap, and, indeed, will not unite with other plants at all. Hence, 
A’cer platandides, according to De Candolle (Physiologie Végétale, 
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 Carya, Rhus, and Aildntus ; or the 
common laurel and the Magnolia 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 Fangi 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 
s 2 
