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 parencliymatous tissue and 

 prosenchymatous tissue. Paren- 



\ ^^ J6N ^ ") cliyma is well illustrated by the 



V\r\ 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 



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- 



FIG. 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 a g a i n s t w o o d - c e 1 1 s. 

 (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. 



