CELLULAR TISSUE. 



361 



and having forms which approximate more or less closely to the 

 globular or ovoidal, which may be considered as their original 

 type. 



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

 when the cells are but loosely aggregated, as in the parenchy- 

 matous (or fleshy) substance of leaves (Fig. 149) ; and it is then 

 only, that the distinctness of the walls becomes evident. When 

 the tissue becomes more 

 solid, the sides of the vesi- 

 cles 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 par- 

 titions appear, except when 

 carefully examined, to be 

 single, instead of being (as 

 they really are) double. Fre- 

 quently it happens that the 

 pressure is exerted more in 

 one direction than in an- 



, , .-, n Section of leaf of Agave, treated with dilute nitric acid, 



Oilier, SO tliat tile lOrm pre- showing the primordial utricle contracted in the interior 

 Sented by the Outline Of the of the cells: a, epidermic cells; 6. boundary cells of 



cell varies according to the ^S*'' "' ^ of parenchyma; d ' lheir primordial 

 direction in which the sec- 

 tion 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 transversely, 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. 152. 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 papyri/era* vertically round and round, with a 

 long sharp knife, so that its tissue may be (as it were) unrolled 

 in a sheet. The shape of the cells, as seen in the rice-paper thus 

 prepared, is irregularly prismatical, as shown in Fig. 150, B ; but 

 if the stem be cut transversely, their outlines are seen to be cir- 

 cular or nearly so (A). When, as often happens, the cells have a 

 very elongated form, this elongation is in the direction of their 

 growth, which is that, of course, wherein there is least resistance. 

 Hence their greatest length is nearly always in the direction of 

 the axis ; but there is one remarkable exception, that, namely, 

 which is afforded by the "medullary rays" of Exogenous stems 

 ( 239), whose cells are greatly elongated in the horizontal direc- 

 tion (Fig. 161, a), their growth being from the centre of the stem 



1 The JEschynomene, which is sometimes named as the source of this article, is an 

 Indian plant, employed for a similar purpose. 



