BARK. 6i 



walls of the cells undergo changes similar to those of the 

 epidermis, and become flexible and elastic, through which 

 water and air permeates with difficulty. The cells usually 

 soon lose their protoplasm and become filled with air. In 

 many cases periderm forms the permanent protective cover- 

 ing of trunks as in the beech and cork-oak, attaining in the 

 latter instance a very considerable thickness ; such rugged 

 structures are popularly and erroneously called bark. When 

 cork is formed in roots it originates from the pericambium, 

 the outside primary cortex being gradually thrown off. 



Bark differs from periderm in not being a specially 

 differentiated tissue, and originates as follows. In many 

 trees layers of periderm form at various depths in the 

 primary cortex, and in some instances extend as far inwards 

 as the fibro-vascular bundles. All tissues external to such 

 plates of cork die on account of being cut off from their 

 food-supply by the impervious layer of cork ; such dead 

 tissue becomes dry and hard, and surrounds the inner living 

 tissues as a protective layer or bark, which' is well developed 

 in the Scotch fir and plane tree. 



Shadowing in of the Vegetable Kingdom. — The 

 conceptions embodied in the idea of plant and animal 

 ai;e not present in the simplest known forms of life, 

 which for the most part consist of exceedingly minute 

 particles of naked protoplasm, portions of which are usually 

 prolonged as cilia or pseudopodia for purposes of locomotion 

 or nutrition ; such organisms present the minimum known 

 amount of differentiation and division of labour, and 

 constitute a group known as the FlagellatcB. The fact of 

 the members of this group mostly requiring organic matter 

 for food along with their aquatic habitat and very rudi- 

 mentary differentiation suggest antiquity, without at the 



