2J2 HISTORV OF BOTAA' V. 



The observations of Priestley, Saussure, and others, aided by the 

 discoveries made in pneumatic chemistry, of the existence of oxy- 

 gen, hydrogen, and catbonic acid gases, formed a new era in the 

 history of vegetable physiology. It was proved that vegetables do 

 ultimately consist of oxijgcn, Jiydrogen, and carbon, sometimes of a 

 small quantity of nitrogen, combined with mineral salts, and often 

 some silex, sulphur, and iron. These elementary substances were 

 found to be diffused through air and water, and the animal and vege^ 

 table substances Which the latter holds in solution : the green parts 

 of vegetables Were observed to exhale oxygen in the light, and car^ 

 bonic acid gas in the dark; and the carbon left by the decomposi- 

 tion of the carbonic acid, was shown to be incorporated into the vege 

 table substance, giving to the wood its strength and hardness. 



The naturalist whose labours, in point of utility, will best bear a 

 comparison with those of LinUceus, is Bernard de Jussieu. He 

 was remarkable for the extent of his knowledge the penetration 

 of his genius, and the solidity of his judgment. He is said to have' 

 been unambitious. The love of truth and science were with him 

 sufficient excitements to the most severe labour. " Many of oui* 

 contemporaries," says Mirbej, " knew this sage ; they say that 

 never have they seen so tnuch knowledge combined with so high a 

 degree of candour and modesty." To this botanist we are indebted 

 for a natural method of classification, superior to those of his pre- 

 decessors. 



Jussieu proposed a method of classing plants according to cer^ 

 tilln distinctions in the seed, which were found to be universal; this 

 was perfected and pubUshed by his nephew, Antoine-Laurent de 

 Jussieu, and is now generally received as the best mode of natural 

 classification which has yet been discovered. This method is called 

 natural, because it aims to bring into groups such genera of plants 

 as resemble each other in medicinal and other properties ; while the 

 system of Linnaeus is called artificial, because, by a certain rule, 

 plants which have no such resemblance in their properties are 

 brought together. We therefore find in one of the Linnasan classes 

 the poisonous flag and the nutritious grass, the grain which supports 

 life and the darnel which destroys it; in another, the healthful pota- 

 to and the poison mandrake, the deadly hemlock and the grateful 

 coriander. Throughout this system we meet with similar contrasts 

 in the qualities of the plants which are collected into the same 

 classes. Nor are their external appearances less unlike ; for here 

 the oleander and pigweed, the tulip and the dock, meet in the same 

 classes. This S3'stem, it should always be remembered, is not the 

 whole science of Botany, but is the key to the natural method, by 

 Which alone, we should find great difficulty in ascertaining the 

 names of plants; it is, as it were, a stepping-stone by which we must 

 ascend to the valuable knowledge which cannot well be reached in 

 any other way. The more practical a botanist becomes, the less 

 need he has for this assistance ; the eye becomes quick to seize on 

 natural characters without reference to the dictionary, as the arti- 

 ficial system is aptly termed. Thus a pupil, in studying a langtiage, 

 may, in time, be able to dispense with his dictionary; though he 

 could not have proceeded, at first, without its assistance. For more 

 particular explanations of Jussieu's method, you are referred to the 

 comparison of that with the method of Linnaeus and Tournefort in 

 the remarks on classification, and to the Natural Orders contained in 

 the appendix. 



Priestley, &c.— Character of Jussieu— Natural method of classing plants. 



