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. 
82: Uses of the Components 
of the Stem. — There is a 
marked division of labor among 
the various groups of cells that 
A ys make up the stem of ordinary 
Fic. 48.— 4A, B, C, D, Isolated Wood- qicotyledons, particularly in the 
Cells and Bast-Cells of Linden. ; : 
A, B, wood fibres ; C, piece of a vessel; stems of trees, and it will be 
D, bast fibre ; E,a partitioned, woody best to explain the uses of the 
Tee ee European ivy. (Much kinds of cells as found in trees, 
rather than in herbaceous plants. 
A few of the ascertained uses of the various tissues are 
these : 
D 
1 See Gregory’s Plant Anatomy, Chapter IV. 
