382 Introduction to Botany. 



Chrom-Acetic Fixative. — Dissolve i gram of chromic acid in 100 

 cubic centimeters of distilled water, and add 0.5 gram of glacial acetic 

 acid. 



When it is desired to study the construction of the protoplasts in 

 embryonic tissues, as in root and shoot tips, or in Algae, Fungi, etc., the 

 fresh material should be submerged in the above killing and fixing solu- 

 tion from I to 2 days. By this treatment the protoplasts will be made 

 to keep the form which they had while living. The fixative should 

 penetrate the material quickly, so that root and shoot tips should be 

 hardly more than 3 millimeters or } inch long. Anthers and ovaries 

 should be cut open at both ends. Ovules should be removed from the 

 ovary with only a narrow strip of tissue adhering to them. Leaves 

 should be cut into narrow strips, not more than 3 millimeters broad, etc. 



A relatively large amount of the fixative should be used ; for example, 

 to a depth of 4 centimeters in a test tube 2 centimeters in diameter, 

 for fixing about 10 root tips. 



After fixing the material tie it up in thin muslin and wash for 6 hours, 

 or over night, in running water, or in water which is frequently changed. 

 Then gradually dehydrate and harden the material by keeping it for 2 

 hours in each of the following grades of alcohol : 20%, 30%, 40%, 50%, 

 60%, 70%. It may then stay in 70% alcohol until wanted. If it is to be 

 kept a long while before using, it would be better to take it from the 

 70% alcohol and preserve it in equal parts of 95 % alcohol, glycerine, and 

 water. If the material is to be stained with safranin (which see), or 

 first sectioned and then stained, it may go directly into the stain from 

 50 % or 70 % alcohol, or from the mixture of alcohol, glycerine, and water ; 

 and then may be mounted for immediate study in equal parts of glycer- 

 ine and water, or it may be mounted permanently in glycerine jelly 

 (which see). 



It is frequently very desirable to embed root and shoot tips, anthers, 

 ovaries, ovules, etc., in paraffin preparatory to cutting very thin and 

 serial sections of them. To learn how to do this read under Imbedding 

 in Paraffin, and Cutting Parafiin Sections. 



Cutting Sections. — Directions for cutting sections free-hand will be 

 found on page 376. 



Sections from hard tissues, such as the shells of nuts, may be cut as 

 thin as possible by means of a hack saw, and then brought to the neces- 

 sary thinness between two oil stones, using water, instead of oil, as a 

 lubricant. 



