CELLULAR TISSUE. 21 
then form continuous tubes of considerable length; retaining 
at the sides the marks of the partitions which formerly divid- 
ed them into short cylindrical cells. These are found chiefly 
in the wood. They were formerly considered as belonging 
to the vascular tissue, and were termed dotted ducts, beaded 
VEssELs. They are now called vastrorm tissue. They 
form the largest tubes found in the vegetable tissue ; it is 
the section of the vasiform tissue which presents the large 
apertures seen in a section of the oak, cane, or other stems 
of plants. The dots on the sides are granules, or are caused 
by a fibre winding round them, adhering intimately at some 
parts, and more loose and somewhat detached at other parts. 
This tissue gives passage to fluids. 
48. Besides the cellular tissue formed of membrane, there 
are some varieties in which a fibre is seen developed on the 
inner surface of the cell. And even fibre alone has been, in 
some instances, observed, coiled up into a sort of cell or 
vesicle. Thus there are three varieties of the cellular tissue 
areey alone—membrane and fibre—fibre.” 
. The cellular tissue formed of membrane = is 
y as the most abundant. The other vagieties, “ particu- 
larly that consisting of fibre alone, are comparatively rare. 
When cellular tissue is spoken of, it is usually the membra- 
nous variety that is meant. 
50. In size, the cells vary considerably —usually from 
1. 300th to 1-1000th part of an inch in diameter. A com- 
mon size is 1-400th. In “plants in general, they are largest 
in the pith or medulla. They are usually large in aquatic © 
plants, being sometimes found there not less than 1-30th 
inch in diameter. In the family Cucurbitacex also they are 
- The tapering cells of the Prosenchyma are sometimes 
not more than 1-3000th inch in diameter. : 
