PARENCHYMA. 43 



fied, as they give rise to kinds of tissue so unlike the ordinary 

 cellular, in outward appearance at least, that they have always 

 been distinguished by special names. We allude particularly to 

 what is called Woody Tissue or Woody Fibre, and Vascular Tissue 

 or Vessels, of various forms. Even since the nature of the vege- 

 table structure has been in a good degree rightly apprehended, 

 these have been considered as essentially different kinds of tissue, 

 of independent origin. But we now know that they are modifica- 

 tions of one common type, the cell, and are produced in the same 

 mode as ordinary cells ; so all the statements of the foregoing sec- 

 tion, in respect to the formation, multiplication, and growth of cells, 

 are equally applicable to these also. Some kinds differ from or- 

 dinary cells in shape alone ; others result from their combination 

 or confluence. This is shown in two ways : first, by noting the 

 intermediate gradations which may be found between every par- 

 ticular sort ; and second, by watching their development and tra- 

 cing them directly from their earliest condition, as ordinary cells, 

 to the peculiar forms they soon assume. The first of the kinds 

 enumerated below is typical cellular tissue ; the second, through a 

 slight change in the development, introduces the special forms. 



51. Parenchyma is the substantive name applied to ordinary 

 membranous cellular tissue in general, such as that which forms 

 the pith of stems, the outer bark, &c. In the most restricted ap- 

 plication, it belongs to such tissue when composed of angular or 

 polyhedral cells (as in Fig. 1-3, 13, &c.) ; the distinctive name 

 of Merenchyma having been proposed for the looser tissues (as in 

 Fig. 7, and in the pulp of leaves and fruits generally), formed of 

 rounded or ellipsoidal cells, that is, where they do not mutually 

 impress each other into plane faces. But this distinction vanishes 

 in the numberless intermediate states; and the name of Paren- 

 chyma is applied to both. That in which the walls barely 

 touch each other, more or less extensively, and leave intervening 

 spaces where the ends or sides are rounded off, is termed by 

 Schleiden incomplete parenchyma. The principal forms of com- 

 plete parenchyma, where the cells are in perfect contact on every 

 side, and the sections are consequently several-sided, are designat- 

 ed by adjective terms ; as the regular, when the cells are dodeca- 

 hedral or cubical ; the elongated or prismatic, when extended lon- 

 gitudinally ; and the tabular, when cubical cells are much flatten- 

 ed ; one kind of which, called the muriform, because the laterally 



