58 



ELEMENTS OF BOTANY. 



in masses to make up the bulk of the plant. Masses of cells 

 which have a common work to do are called tissues.^ Two 

 of the most important forms of tissue are parenchyma and 

 prosenchyma. Parenchyma is found in the seed, in the bark 

 (constituting the greater portion of all young bark), in the 

 medullary rays and the pith, and in the leaf. Parenchyma 

 cells are usually roundish or somewhat cubical or twelve-sided 

 in shape. 



From the fact that a sphere surrounded - by other spheres 

 is touched by twelve others, parenchyma cells, which begin 

 . their existence in a somewhat* 



globular form, often end by 

 growing approximately twelve- 

 sided from the pressure of their 

 neighbors. Prosenchyma cells 

 are long, often thick-walled, 

 and interlock at the ends, so as 

 to leave but few and small in- 

 tercellular spaces. They form 

 the fibrous part of bark and of 

 most kinds of wood. 



82l Uses of the Components 

 of the Stem. — There is a 

 marked division of labor among 

 the various groups of cells that 

 make up the stem of ordinary 

 dicotyledons, particularly in the 

 stems of trees, and it will be 

 best to explain the uses of the 

 kinds of cells as found in trees, 

 rather than in herbaceous plants, 

 the ascertained uses of the various tissues are 



fc 



FiO. iS.—A, B, C, D, Isolated Wood- 

 Cells and Bast-Cells of Linden. 



A, B, wood fibres ; C, piece of a vessel ; 

 D, bast fibre ; E, a partitioned, woody 

 fibre from European ivy. (Muoli 

 magnified.) 



'A. few of 

 these : 



> See Gregory's Plant Anatomy, Chapter IT. 



