44 THE ELEMENTARY STRUCTURE OF PLANTS. 



compressed cells appear in the magnified section like courses of 

 bricks in a wall, is seen in the silver-grain of wood (Fig. 20, c). 



52. Prosenchjma is the general name to designate tissues formed 

 of elongated cells, with pointed or conical extremities ; their nar- 

 rowed ends overlapping and thus filling up the intervening spaces 

 which must otherwise exist. Every gradation may be traced be- 

 tween this and incomplete parenchyma. As to length, such cells 

 vary from fusiform, or spindle-shaped, only three or four times 

 longer than broad, to tubular, and to tubes so long and narrow that 

 they are commonly called fibres. As to their extremities, they 

 are often sp blunt, and applied to each other with such moderate 

 obliquity, that they are more properly said to be placed end to end 

 than side by side ; while, again, precisely similar cells, sometimes 

 even in the same bundle, exhibit flattened ends resting directly one 

 over the other.* Nor can we draw any fixed line of distinction 

 from the thickness of the walls. Indeed, no one can spend a few 

 hours over the microscope in diligently examining the tissues of 

 two or three of the commonest plants, without perceiving that 

 there is no essential difference between cellular and 



53. Woody Tissue, (Pleurenchyma of Meyer and Lindley. Woody 

 Fibre of the older authors). Wood, which makes up so large a 

 part of trees and shrubs, and a distinguishable portion in all Phce- 

 nogamous (110) herbaceous plants, is wanting in Mosses and plants 

 of still lower grades, such as Lichens, Sea-weeds, and Fungi. 

 That is, in the latter there" is no formation corresponding to the 

 wood of higher plants, although many of them exhibit, at least in 

 certain parts, prosenchymatous cells, and others drawn out into 

 tubes or hollow fibres of greater length and tenuity than are those 

 of ordinary wood ; such, for instance, as the interlaced fibrous tis- 

 sue of Lichens (Fig. 15). Nor, on the other hand, does the proper 

 woody system of trees (except in the Pine Family) consist entirely 

 of that form which has received the special name of woody tissue, 

 but three or four other sorts are variously intermingled with it. 

 Indeed, there are some trees whose wood is almost entirely com- 

 posed of true parenchemytous, or of large dotted (58) cells ; while 

 in stone-fruits, and many like cases, common parenchemytous 



The forming woody tissue, as seen in a germinating plant or young root- 

 let, consists of prismatic cells, with square ends ; as these lengthen, their ends 

 push by each other, and so become oblique and wedged together, or converted 

 into prosenchyma. 



