INTRODUCTION. 



XI 



subject to endless variation even in different parts 

 of the same plant. They contain watery juices, 

 true sap, sugar, gum, resin, &c. : ^ ^ 

 sometimes they appear empty, or ' ^ - 

 are filled only with air. They vary 7 ~ ^ 

 in consistence, from the soft pulp T> 

 of the peach, to the stony nut which ^ y-^ 



it encloses. They have the powder ^ 



P . n • 1 /? V CELLULAR TISSUE. 



01 transmittmg liuid irom one to 

 another, though no pores have been to a cer- 

 tainty discovered in the membrane of which their 

 sides are composed. 



The vascular system comprises all those parts 

 of plants which are known in common language 

 by the names of fibres, nerves, veins, tubes, &c., 

 all of which may be classed under the term ves- 

 sels." They are found in the root, trunk, branches, 

 leaves, and fiowers of trees, collected into bundles, 

 but often so closely connected as to have the 

 appearance of simple tubes. They are either 

 of an uniform continuous substance throughout 

 their whole length, when they constitute ivoody 

 fibre; or are composed of a thread, or collection 

 of threads, twisted spirally, so as to form a tube, 

 and capable of being unrolled with elasticity. 

 If a leaf of the strawberry, or young twig of 

 the cornel, be broken, and the parts gently torn 

 asunder, they will be discovered like fine cob- 

 webs uniting the ragged edges. Under the micro- 

 scope, if stretched, they resemble a corkscrew ; 

 but if examined at rest, their appearance may 

 be compared to that of a bell-spring, that is 

 a coil of wire wound round a cylinder which 

 has been afterwards removed. They are called 

 spiral vessels, A third form of vessels is the duct^ 



