94 FOUNDATIONS OF BOTANY 
(see Fig. 66, t’’). The cardboard represents a part of the 
cell-wall common to two adjacent cells, and the watch- 
glasses are like the convex border bulging into each cell. 
Fic. 66. — Longitudinal Radial Sec- 
tion through a Rapidly Growing 
Young Branch of Pine. 
t,t',t”, bordered pits on wood-cells ; 
st, large pits where medullary 
rays lie against wood-cells. 
(Much magnified.) 
times longer than wide, 
When the cells grow old the 
partition in each pit very com- 
monly breaks away and leaves 
a hole in the cell-wall. 
106. Tissues. — A mass of 
similar cooperating cells is called 
a tissue! Two of the principal 
classes which occur in the stem 
are parenchymatous tissue and 
prosenchymatous tissue. Paren- 
chyma is well illustrated by the 
green layer of the bark, by wood 
parenchyma, and by pith. Its 
cells are usually somewhat 
roundish or cubical, at any rate 
not many times longer than wide, 
and at first pretty full of proto- 
plasm. Their walls are not 
generally very thick.2 Prosen- 
chyma, illustrated by hard bast 
and masses of wood-cells, con- 
sists of thick-walled cells many 
containing little protoplasm and 
often having little or no cell-cavity. 
As a rule the stems of the most highly developed plants 
owe their toughness and their stiffness mainly to prosen- 
1 See Vines’ Students’ Text-Book of Botany, London, 1894, pp. 131-144. 
2 Excepting when they are dead and emptied, like those of old pith. 
