CELLULAR TISSUE OF FLOWERING PLANTS. 



387 



284. As a general rule, the rounded shape is preserved only 

 when the Cells are but loosely aggregated, as in the Parenchy- 

 matous (or pulpy) substance of Leaves (Fig. 197), 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 them and to bring them into 

 close apposition ; and they then adhere to one another in such a 

 manner that the partitions appear, except when carefully examined, 

 to be single instead of double, as they really are. Frequently it 



Fig. 197. 



Section of Leaf of Agave, treated with dilute nitric acid, show- 

 ing the Primordial Utricle contracted in the interior of the cells : 

 — a, Epidermic cells ; b, boundary-cells of the Stoma ; c, cells of 

 Parenchyma ; d, their Primordial Utricles. 



happens that the pressure is exerted more in one direction than in 

 another, so that the form presented by the outline of the cell varies 

 according to the direction in which the section is made. This is 

 well shown in the Pith of the young shoots of Elder, Lilac, or 

 other rapidly -growing trees ; the cells of which, when cut trans- 

 versely, generally exhibit circular outlines, whilst, when the section 

 is made vertically, their borders are straight, so as to make them 

 appear like cubes or elongated prisms, as in Fig. 198. A very 

 good example of such a Cellular Parenchyma is to be found in the 

 substance known as Rice-paper; which is made by cutting the 

 herbaceous stem of a Chinese plant termed Aralia papyrifera* 

 vertically round and round with a long sharp knife, so that its 



* TheJEschynomene, which is sometimes named as the source of this 

 article, is an Indian plant employed for a similar purpose. 



C C 2 



