PAKENCHYMA AND PROSENCHYMA. 



41 



ordinary cells, to the peculiar forms they soon assume. In enumer- 

 ating the kinds of vegetable tissue, we commence with cellular tissue 

 strictly so called, or 



\ 51. Parenchyma. This is the distinctive name for ordinary mem- 

 branous cellular tissue in general, such as that which forms the pith 

 , of stems and their outer bark. In the most restricted application, it 

 belongs to such tissue when composed of angular or polyhedral cells 

 (as in Fig. 1-3, 9, &c.) ; the 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 Parenchyma is applied to both. That in 

 which the walls touch each other, more or less, and leave interven- 

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

 Schleiden incomplete parenchyma ; and that in which the cells are in 

 perfect contact on every side, complete parenchy- 

 ma. The latter is regular, when the cells are 

 dodecaliedral or cubical ; elongated or prismatic, 

 when extended longitudinally; and tabular, when 

 cubical cells are much flattened; one kind of 

 which, called the muriform, because the laterally 

 compressed cells appear in the magnified section 

 like courses of bricks in a wall, is seen in the 

 silver-grain of wood (Fig. 192). 

 \ 52. Prosenchyma is the general name for tissues 

 formed of elongated cells, especially those with 

 pointed or oblique extremities. Every gradation 

 may be traced between this and parenchyma. As 

 to length, such cells vary irova. fusiform, or spindle- 

 shaped, only three or four times longer than broad, 

 to tuhilar, and to tubes so long and narrow that 

 they are commonly called fibres. The most char- 

 acteristic form of prosenchyma is 

 \ 53. Woody Tissue. {Pleurenchyma of Meyer and 

 Lindley. Woody Fibre of the older authors.) 

 Wood, which makes up so large a part of trees 



a--0 



FIG. 46. Some wood-cells of the Plane-tree or Buttonwood, highly magnified : u., thin 

 Bpots in the walls, looking like holes ; on the right-hand side, where the walls are cut through, 

 these {b) are seen in profile. 



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