viri.l 



MINUTE STRUCTURE. 



109 



upon the inside of their walls. In Balsam this thickening 

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

 of Teak or 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 dangerous acid, you may dissect it in the same 

 way as you did the Balsam. You will find the vessels 

 which it contains more or less like those in Fig. 86. The 

 larger vessels of Teak and Oak wood differ from those of 

 Balsam merely in the thickening on the inside of the vessel 



Fig. .85. Spiral and annular vessels, 

 associated with long narrow cells. 



Fig. 86. Dotted duct, associated with 

 long, tapering, thick-walled cells. 



being more uniformly spread over the wall, omitting only 

 minute spots, which look like holes or pores through the 

 wall of the vessel. Such 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 Balsam that of the 

 following tissues, selected as well suited to show different 

 modifications of cells and vessels, because easily obt(\ined 

 and requiring little preparation. 



