FORMATION OF CELLS— TISSUES 



315 



by projecting rays, and in contact only by the extremities of such 

 rays, so aa to leave large irregular spaces between them. 



c. Regular or Polyhedral Parenchj'ma. — This is formed of 

 polyhedral cells, the faces of which are frequently nearly equal, 

 and so combined as to leave very small interspaces. It is 

 commonly found in the pith of plants. 



d. Elongated Parenchyma. — This is composed of cells elon- 

 gated in a longitudinal direction so as to become cylindrical. It 

 occurs frequently in the stems of Monocotyledonous plants. 



c. Tabular Parenchyma. — This consists of tabular, closely 

 adherent cells. It is found in the epidermis and other 

 external parts of plants (fig. 675, e). A variety of this liind of 



Fig. 673. 



Fig. 675. 



Fig. 672. 



^^^ 



Fig. 672. Round or oval pareucliyma. In two of tlie cells a nucleus with a 



nucleolus may be seen. Fif;. 673. Stellate parenchyma, composed 



of steUate cells with three-cornered intercellular spaces. Fig. 674. 



Muriform parenchyma. Fig. 675. Transverse section of the petiole 



of a species of Begonia, e. Epidermis with cuticle above and hypoderma 

 below, the latter formed of coUenchymatous cells cl, cl, with thickened 

 angles V, V. chl. Chlorophyll granules, p. General parenchyma, below 

 hypodeiTna. After Sachs. 



parenchyma is called muriform, because the cells of which it is 

 composed resemble in their form and arrangement the courses 

 of bricks in a wall [fig. 674) : this variety occurs in the 

 medullary rays of the stems of Dicotyledons and in corky 

 formations. 



Such are the commoner varieties of parenchyma, all of 

 which are connected in various ways by transitional forms ; but 

 other special kinds also occur. In the tissue which is placed 

 below the epidermis of plants, which has been termed the 

 hypoderma, we sometimes find the parenchyma composed of 

 cells which are especially thickened at their angles {fig. 675, 



