VIII. J 



TISSUES. 



93 



some cases the common intervening membrane disap- 

 pears, and so direct communication becomes established 

 between the cells. In Rhubarb this thickening usually 

 takes the form of a spiral fibre ; but if you boil a bit of 

 oak-wood, half the size of a pea, in a few drops of nitric 

 acid for a few seconds, it will become white and soft, and 

 after washing it in water two or three times to remove the 

 corrosive acid you may dissect it in the same way as you 

 did the Rhubarb. You will find the vessels which it con- 

 tains more or less like those in the cut. The larger vessels 



FIG. 65. Vessels from the petiole 

 of Rhubarb, magnified. 



FIG. 66. Thin section of Oakwood, 

 magnified, showing a dotted vessel, 

 between long wood-cells. 



of Oak-wood differ from those of Rhubarb merely in the 

 thickening on the inside of the vessel being more uni- 

 formly spread over the wall, omitting only minute spots, 

 which look like holes or pores through the wall of the 

 vessel. Such pitted or dotted vessels are very common 

 in wood, and may be easily observed by making very thin 

 slices of the wood lengthwise with a sharp razor. 



Plants, then, are built up of cells, or of cells and vessels ; 

 the latter originating from cells. 



4. Compare with the structure of Rhubarb and Oak- 

 wood that of the following tissues, selected as well as suited 

 to show different modifications of cells and vessels, 

 because easily obtained and requiring little preparation. 



Pulp of ripe Strawberry. Large, thin-walled cells. 



