MICROSCOPIC OBJECTS. 71 



plants, owing to imperfect notions of the form of the 

 organs themselves. 



Cellular Tissue — is the first and most generally developed 

 simple form of vegetable life. No plant, whether belong- 

 ing to the highest or the lowest tribe, is without it ; many 

 are destitute of any other kind of tissue, as the Lichens, 

 and some fresh water Algae. It consists of distinct closed 

 vesicles or cellules of various forms and varied contents ; 

 they cohere together, and the consequent pressure in 

 some respects modifies their form. They differ greatly 

 in dimensions ; the largest are found in aquatic plants, 

 of which the Nitella is a good example, the stem being 

 composed of a single cell, sometimes four inches long. 

 The general diameter of ordinary ceUules varies from ^^(jth 

 to 3-^o^h of an inch. The plants named in the Catalogue 

 under this heading (Cellular Tissue) furnish most of the 

 varieties of this kind of tissue. 



Elder Pith, 8^c. — The pith of exogenous trees and 

 plants is situated in the centre of the stem, and consists 

 of cellular tissue. In the elder and fig it is large in pro- 

 portion to the diameter of the stem ; in the oak and elm 

 small. In the willow it is compact ; in the walnut loose 

 and interrupted. In young trees the cellular tissue is also 

 to be found in the root. The form and diameter of the 

 cells vary in different specimens. The" membrane of 

 which they are composed is always colourless and desti- 

 tute of perforations. The petals of flowers ai-e mostly 

 composed of cellular tissue ; their briUiant colours arise 

 from the fluid contained within the ceUules. 



H 2 



