20 CELLULAR TISSUE. 
In the medulla or pith, where the cells are exposed to conside- 
rable pressure, but that pressure is tolerably equal on every 
side ; each cell being pressed upon by twelve surrounding 
cells, its sides are flattened at the points of contact, and it 
becomes the plane twelve sided figure called a rhomboi- 
dal dodecahedron—a figure which presents a hexagonal sec- 
tion. In the wood and bark, which contain woody tissue 
and vascular tissue as well as cells, and where the pressw 
is chiefly lateral, the cells become elongated, and are streteh- 
ed out into a tubular or cylindrical form, frequently tap 
at the extremity. 
45. In many cases the cells or vesicles have a fibre wo 
spirally round their inner surface. The coils of the fibre 
sometimes close to each other, sometimes a little apart 
some instances the coils cross each other, and they are oce 
sionally branched. The coils of the fibre are som 
closely united at one part, and separate at others, 
gives the appearance of slits, or dots on the side of the 
and Mr. Slack, who has studied with great care the 
with streaks, &c. on their sides, refers the appearances of 
or pores in the sides of the cells, to the different way 
_ which the vegetable fibre passes over the surface of the 
eo e, closely incorporated with it at some places, 
at others, crossing and interlacing, &e. 
46, The cellular tissue was formerly called pulp or at 
= chines: To the elongated tapering form of cells which 
_ lap each other at the extremities, found in the wood 
_ bark, Livx applies the term prosenchyma. These ar 
called fusiform cells or closters. He confines t 
parenchyma, to the cells of the leaves, pith, medu 
_ &e., in which, though occasionally elongate 
taper and overlap at the extremities, but fit 
_ compactly to each other. And all the soft su 
oes, heghierarmemnre nia xp 
4 In some forms of elongated 
