SUBSTANCES PRODUCED BY PLANTS. 53 



C) I . When the wood of a tree is examined by a powerful 

 microscope, it is observed to consist of layers, one of which is 

 composed of the cellular fibre just described, while the others 

 consist of an incrusting substance which diflfers from it in 

 containing more cai'bon, and hydrogen in a larger amount, 

 than would combine with the oxygen which it contains, to 

 form water. Many interesting researches have lately been 

 made respecting the composition of these layers, but the 

 inquiry is of peculiar difficulty and must be regai'ded as only 

 commenced. 



62. When a piece of wood is placed in vitriol, it is blackened 

 and assumes the appearance of charcoal ; thus, you may have 

 observed that a piece of cork placed in a phial containing vitriol 

 gives a deep black colour to the entire liquid. The strong 

 acid decomposes the woody matter and unites with its hydro- 

 gen and oxygen, the elements of water which it contains, 

 while its carbon is set free. 



63. The greater part of the heart- wood and bark of trees 

 is composed of woody fibre. It constitutes 50 per cent of the 

 weight of barley straw dried in the air, and about 80 per 

 cent of the weight of the dried straw of the flax plant. In 

 the root crops, however, its amount is but small, the white 

 turnip in its fresh state containing only three per cent of it, 

 hut as the plants grow old its quantity increases, so as to 

 render them stringy and unfit for the table. 



64. Starch. Next to woody fibre, starch is one of the 

 most common forms into which plants convert the materials 

 derived from the air. It can be readily procured and its 

 properties examined. 



If we grate a potato upon a common grater placed over 

 a basin, and allow a stream of water to fall upon the grater, 

 so long as it flows through milky we perform a mechanical 

 analysis of that root. 7\.t the bottom of the basin a white 

 j)0wder is gradually deposited from the water. This powder 

 is the well-known substance Starch, which is met with in 

 greater or less quantity in every vegetable, and which under 

 different names — as arroiv root when procured from the roots 

 of a West Indian plant, sago when extracted from the pith of 

 a species of palm, and/arma when manufactured from pota- 

 toes, — is used for food in almost eveiy country. No matter, 

 however, from what plant or in what climate starch has been 

 produced, the chemist recognises its composition to be the 



