92 CELLS AND VESSELS. [CHAP. 



end of a needle and put it upon a slip of glass, adding a 

 drop of water. If you have a thin glass cover put it over 

 the drop, gently letting one side rest first on the slip as 

 you put it down, so as to push out the air-bubbles, which 

 are apt to get entangled, and which look like round balls 

 with black sides when magnified. When you look at the 

 Rhubarb under the microscope you are pretty sure to 

 find a number of bodies resembling those represented in 

 the cut. If you do not find them, try another morsel 

 until you succeed. These bodies are called cells. They 

 are hollow sacs, each filled with fluid. Now, of cells 

 more or less like these, differing principally in size, in 

 relative length and breadth, and in the thicknesses of 

 their sides, every part of every plant is composed. All 

 the organs are built up of these minute cells. 



3. But take now a very small bit of one of the fibrous 

 strings of the boiled Rhubarb. Place it in a drop of 

 water, and, with a couple of needles, one in each hand, 

 separate it into what seem, to the naked eye, to be its 

 constituent fibres, just as you would separate a morsel of 

 string into its finest threads. When you have got it 

 dissected out, put a cover on as before, and examine it 

 under the microscope. You will probably find here, 

 besides numerous cells of various lengths, some long 

 tubes, with their sides (walls) curiously marked with 

 delicate fibres, usually arranged in a spiral direction, 

 twisting round and round inside the tube the coils some- 

 times very close, sometimes loose; or you may find the 

 fibre in the form of separate rings in the inside of the 

 tubes. These tubes are called vessels. They originate in 

 this way. A number of cells, such as we saw before, 

 standing one over the other in a row, have the partitions 

 which separate them more or less completely removed, so 

 that the row of cells becomes open all through. We 

 have then a true vessel. Vessels are almost invariably 

 marked either by a spiral, netted, pitted, or ring-like 

 thickening of their walls. In pitted vessels, as in Oak 

 (Fig. 66), the minute circular or oblong "pits" are 

 spaces left unthickened on the original cell-wall. These 

 unthickened spaces in contiguous cells, when not spirally 

 disposed in each cell in the same direction and so neces- 

 sarily crossing, are strictly opposite to each other; and in 



