QHXP, I. CELLULAR TISSUE. » 



end, and unquestionably adhering by one of their ends to the 

 inside of the sac ; others from polygons passed to a spherical 

 form in rounding off their angles. As they grew older, odier 

 very important changes took place in certain cells of the ordi- 

 nary structure, which had not previously undergone any alter- 

 ation : in each of these there appeared three or four rings 

 placed parallel with each other, adhering to the membrane, 

 from wliich diey were distinguished by tlieir opaqueness; 

 these were altogether analogous to annular ducts. The cells 

 become tubes did not at first differ from other cells in any 

 thing except their form ; their sides were uniform, thin, colour- 

 less,'and transparent; but they soon began to thicken, to lose 

 their transparency, and to be marked all round from end to 

 end with two contiguous parallel streaks disposed spirally. 

 They then enlarged, and their streaks became slits which cut 

 the sides of the tubes from end to end into two threads, whose 

 circumvolutions separated into the resemblance of a gun- 

 worm.' In these cases there can, I think, be little doubt that 

 the changes witnessed by Mirbel were chiefly owing to the 

 developementof a spiral thread in the inside of the tissue; he, 

 however, did not consider it in that light. 



But however clearly the origin of the different forms of tissue 

 may be shown to be identical, it is obviously important to dis- 

 tinguish them for practical purposes. I shall therefore pro- 

 ceed henceforward to speak of them as if they were totally 

 distinct in their origin. 



Sect. I. Of Cellular Tissue. 



Cellular, Utricular, or Vesicular tissue, generally, con- 

 sists of little bladders or vesicles of various figures, adhering 

 together in masses. Occasionally it seems to be composed of 

 fibre only, unconnected by membrane. It is transparent, and 

 in all 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 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 hexagonal 



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