448 THE ANATOMY OF WOODY PLANTS 



and should have their faces corresponding with the transverse, 

 radial, and tangential planes of structure of the wood. 



When the wood is softer, after the preliminary boiling, which 

 should not be shortened, the pieces of larger size, on account of the 

 greater ease of manipulation, are transferred to acid which has 

 either been diluted with water or been used once before on harder 

 material. In this they are left for a shorter time than is necessary 

 to effect the softening of very hard tissues, such as heavy tropical 

 woods or the shells of nuts. For poplar or fir wood a week is quite 

 long enough to bring the tissues into good condition for sectioning. 



When complete organs are to be sectioned, such as stems, roots, 

 leaves, etc., a preliminary fixation in some preserving fluid is 

 necessary. After the material has been freed of the fixing agent, 

 in case this is any fluid other than alcohol, it is run up into strong 

 alcohol and then freed of air by means of a good air pump. The 

 air must be removed as completely as possible so that the pieces 

 will sink even in water. After the pumping has been completed, 

 the specimens are transferred to water and left until they sink. 

 If the parts are rather delicate, it is well to carry on the process 

 of demineralization in weak alcohol instead of water to avoid 

 maceration. In most cases, however, if the material has been 

 well fixed in a suitable reagent, no appreciable injury is caused 

 either to the cell wall or to the protoplasm by the use of hydro- 

 fluoric acid. 



Naturally the demineralization of plant tissues of whatever 

 category through the agency of hydrofluoric acid must be carried 

 on in receptacles made of wax (either hard paraffin or beeswax 

 will serve) ; or, in case glass bottles are used, these must be coated 

 both internally and externally with wax. This is necessary to 

 avoid the destruction of the container. Gutta-percha or hard- 

 rubber bottles are sometimes sold for the purposes indicated, but 

 they are not so resistant to acid as wax and are unnecessarily 

 expensive. Further, they cannot, like wax, be used with chlorates 

 in certain procedures to be later described. Whatever be the 

 nature of the material to be softened, it must, after remaining 

 sufficiently long in hydrofluoric acid, be washed entirely free of this 

 reagent in running water. The washing is best effected in the case 



