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. 



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. 1 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 

 parenchvma, and by pith. Its 

 ce ^ s 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 

 times longer than wide, 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- 



FlG. 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.) 



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. 



