446 THE ANATOMY OF WOODY PLANTS 



in the protoplasm of the cells which is to be removed, after the 

 material has been kept in strong alcohol for some time, by the addi- 

 tion of crystals of metallic iodine until the alcohol ceases to lose 

 the brown color imparted by the iodine. In all cases the preserved 

 material should finally be brought into strong alcohol as a prelimi- 

 nary to further treatment. Where the parts are delicate, such as 

 small roots, slender herbaceous stems, or the organs of aquatics, 

 alcohol of full strength should be reached by gradual stages, through 

 30, 50, 70, and 90 per cent grades. 



THE MACERATION OF MATERIAL 



It is often important, particularly in the case of the more 

 resistant tissues, to separate the cells from one another by the 

 dissolution of the cement substance or middle lamella. This end is 

 sometimes attained by the use of nitric acid and a chlorate. Such a 

 procedure, however, is unnecessarily violent and causes a consid- 

 erable amount of injury to the cells themselves. A better method 

 is to use chromic acid in conjunction with nitric acid, and to warm 

 in a paraffin bath, if it is desirable to hasten the process. Macera- 

 tion in the cold, however, often gives much better results. The 

 strength of chromic acid varies according to the material. From 

 5 to 10 per cent strength of the two acids ordinarily suffices to 

 bring about the necessary degree of maceration. The material 

 after soaking for some time in the macerating fluid may be washed 

 in repeated changes of water and then gently scraped with the point 

 of a needle or scalpel. As a result of this procedure masses of ele- 

 ments may be removed which are readily teased apart with needles 

 or separated by tapping on the cover glass. 



In the case of carbonized material, such as, for example, charred 

 wood in coal, commonly but not very aptly designated "mother 

 of coal," boiling in nitric acid often yields quantities of isolated 

 tracheids and other elements of the wood showing all the details 

 of structure. The action of the hot nitric acid in this case is a 

 double one, since it not only affects the isolation of the elements, 

 but also bleaches the black hue of the coal to a light brown. In 

 this manner details of structure are made apparent. Spores are 

 likewise isolated from coal by this method. A more powerful action 



