686 MICROSCOPIC STRUCTURE OF PHANEROGAMIC PLANTS 



support to their softer textures ; and the small proportion of their 

 substance which it forms is at once seen in those beautiful ' skeletons ' 

 which, by a little skill and perseverance, may be made of leaves, 

 flowers, and certain fruits. All the softer and more pulpy tissue 



of these organs is com- 

 posed of cells, more or less 

 compactly aggregated to- 

 gether, and having forms 

 that approximate more or 

 less closely to the globu- 

 lar or ovoidal, which may 

 be considered as their 

 original type. 



As a general rule, the 

 rounded shape is pre- 

 served only when the cells 

 are but loosely aggre- 

 gated, as in the parenchy- 

 matous (or pulpy) sub- 

 stance of leaves, which 

 often forms a distinct 

 layer known as the 

 ' spongy parenchyme ' 

 immediately beneath the 

 epiderm of the upper sur- 

 face (fig. 524) ; and it is then only that the distinctness of their 

 walls becomes evident. When the tissue becomes more solid, the 

 sides of the vesicles are pressed against each other, so as to flatten 



FIG. 524 Section of leaf of Agave, treated with 

 dilute nitric acid, showing the protoplasmic con- 

 tents contracted in the interior of the cells ; a, 

 epidermal cells b, guard-cells of the stomate ; 

 c, cells of parenchyme; d, their protoplasmic 

 contents. 



FIG. 525. Sections of cellular parenchyme of Aralia, or rice-paper plant 

 A, transversely to the axis of the stem ; B, in the direction of the axis. 



them and to bring them into close apposition, and then the cavities 

 of adjacent cells are separated by a single partition wall. Fre- 

 quently it happens that the pressure is exerted more in one direction 

 than in another, so that the form presented by the outline of the cell 



