CHAP. I. 
CELLULAR TISSUE. 
9 
Mirbel were chiefly owing to the developement of a spiral 
thread in the inside of the tissue ; he, however, did not con- 
sider it in that light. 
But although the origin of the different forms of tissue 
may be shown to be identical, it is obviously important to 
distinguish them for practical purposes. I shall therefore pro- 
ceed henceforward to speak of them as if they were distinct 
in their origin. 
Sect. I. Of Cellular Tissue^ or Parenchyma. 
Cellular, Utricular, or Vesicular tissue, generally 
consists of little bladders or vesicles of various figures, ad- 
hering together in masses. It is transparent, and in most 
cases colourless : when it appears otherwise, its colour is caused 
by matter contained within it. 
If a thin slice of the pith of elder, or of any other plant, 
be examined with a microscope, it will be found to have a 
sort of honeycomb appearance, as if there were a number of 
c 
hexagonal cavities, separated by partitions {Jig> 3.). These 
little cavities are the inside of bladders of cellular tissue ; and 
the partitions seem to be caused by the cohesion of their sides, 
for if we boil the pith for a short time, the bladders readily 
separate from each other. In pulpy fruits, or in those which 
have their cellular tissue in a loose dry state when ripe, the 
bladders may be readily separated from each other without 
boiling. It was formerly thought that cellular tissue might 
be compared to the air bubbles in a lather of soap and water ; 
