Structure of Vegetables . 
195 
Wood *— The wood, which lies immediately under* 
the bark, is composed of numerous concentric layers 
which increase with the age of the plant, and may be 
separated into thinner layers, which consist of longi- 
tudinal fibres. The wood which is next the bark is 
softer and whiter, and is hence called alburnum or 
white-wood, and known to workmen by the name of 
Sap ; the interior part of the trunk is browner and hard- 
er, and is denominated the perfect wood. In the la- 
burnum the concentric layers which constitute the al- 
burnum are yellowish, and the perfect wood is brown. 
A transverse section of lignumvitae affords a good ex- 
ample of the same diversity of appearance. 
Pith .— The medulla or pith occupies the centre of 
the trunk or stem of the plant, and in growing stems 
or branches is a tolerably firm juicy substance ; but 
when the same parts are fully grown, it becomes ex- 
tremely light and cellular. Many of the grasses and 
umbelliferous plants have always hollow stems, lined 
only with a thin smooth coating of pith. Of the na- 
ture and functions of the pith, physiologists have enter- 
tained very different opinions ; some have supposed that 
it performs no important office in the economy of 
plants, and others regard it as the seat of life and the 
source of vegetation ; but whatever be its nature and 
functions, as it is most vigorous and abundant in young 
and growing branches, it cannot be doubted that it per- 
forms some essential part. Mr Knight supposes that 
the pith is a reservoir of moisture to supply the leaves 
when an excess of perspiration takes place ; but it has 
been remarked by Dr Smith, that all the moisture in 
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