90 CHEMICAL CONSTITUTION OF VEGETABLES. 



agents. But treatment of different kinds, which it is not necessary 

 to enter upon in this place, is required to procure the substance in a 

 state of perfect purity.* 



The facts which have just been exposed, in regard to the chemi- 

 cal composition of wood, corroborate the observations of physiolo- 

 gists. We now understand much better than we did formerly the 

 changes which the cells of vegetables experience as they grow and 

 become aged : it is by the appearance of the incrusting woody mat- 

 ter that their walls, thin, transparent, and colorless at first, get thick, 

 become opaque, and acquire consistence. By means of the dissec- 

 tions effected by M. Payen with the aid of purely chemical means, 

 we may obtain assurance that the tissues of all vegetables, whether 

 phoenogamous or cryptogamous, may be reduced to a single substance, 

 cellular tissue, having an invariable composition, and forming the 

 vesicles or bladders of the cellular mass of plants. 



This matter exists nearly in an isolated state in the thick walls of 

 the cells of the perisperms of various seeds, those of the date for 

 example. From the microscopic researches of M. Payen and A. 

 Brongniart, it appears that the matter which is added to the young 

 cells is not deposited upon the inner surface of their walls, but that 

 it penetrates and insinuates itself into their tissue. The relation of 

 the cellular to the woody matter in the development of the walls of 

 cells varies very much, some perisperms containing nothing but pure 

 cellular tissue, while the stony concretions of the pear and of cork 

 consist almost entirely of incrusting woody matter. 



Wood, in the general acceptation of the word, is the solid part of 

 the trunk and branches ; the properties and aptitudes of the substance 

 vary greatly, according to the plant which has produced it. Wood 

 is of higher density than water, and if it floats in this fluid it is only 

 because of the air with which its pores are filled. Saw-dust, chips, 

 and larger pieces of wood sink when the air which they contain is 

 expelled and replaced by water. The specific gravity of the white 

 woods, such as those of the willow and pine, is about 1.46, that of 

 the heaviest woods, such as those of the oak and the beech, 1.53. 



DENSITY or DIFFERENT KINDS OF WOOD ACCORDINO TO BRI8S0N. 



Pomegranate 1.35 



Guaiac, Ebony 1.33 



Box 1.32 



Oak of 60 years old, the heart. .. 1.17 



Medlar 0.94 



Olive 0.92 



Spanish Mulberry . 0.89 



Orange 0.70 



Quince 0.70 



Elm, the trunk 0.67 



Walnut 0.67 



Pear 0.66 



Spanish Cypress 0.64 



Lime 0.60 



Beech 0.85 i Hazel 0.60 



Ash 0.84 Willow 0-53 



Hornbeam 0.80 Thuya 0..56 



Yew 0.80 I Pine 0-55 



Apple 0-79 1 Spanish white poplai 0.52 



Plum o.: 



Maple 0.75 



Cherry 0.75 



Pine 0.49 



Poplar 0.38 



Cork 0.24 



* For an account of these, see Payen in proceedings of the Academy of Science^ 

 voL viL p. 1055. 



