114 Methods in Plant Histology 



over the surface, and then wipe it off with a clean finger until only a 

 scarcely perceptible film remains; then add several drops of distilled 

 water and float the sections or ribbons on the water. Warm gently 

 until the paraffin becomes smooth and free from wrinkles. Be care- 

 ful not to melt the paraffin, for the albumen of the fixative coagulates 

 with less heat than is required to melt the paraffin. If the paraffin 

 should melt, run some cold water under it, and transfer the ribbon 

 to another slide, prepared with fixative and water. It is a very good 

 plan to put the slide on a metal bath or warming plate, like that 

 shown in Fig. 10. After the sections have become smooth, remove 

 the surplus water and leave them on the bath with a couple of thick- 

 nesses of blotting paper under them for 3 or 4 hours, or, better, over 

 night. If the fixative is used alone, as is often the case when sections 

 are very thick, none of this delay is necessary, since the sections 

 are merely laid upon the fixative and pressed down gently with the 

 finger. 



Land's Fixative. -Mayer's fixative is so easily prepared and 

 it keeps so well that it is in universal use ; but, in many cases, it will 

 not hold the section to the slide. Moss archegonia and moss capsules 

 are likely to wash off, especially if cut rather thick. Large sections 

 of cones of conifers are almost sure to float off as soon as the slide 

 comes into the xylol or alcohol. Sections of ovules of cycads, as 

 soon as they attain a length of 1.5 to 2 cm., are likely to wash off. 

 For handling these more difficult cases, Dr. Land devised a fixative 

 which has proved satisfactory, even in such extreme cases as sections 

 of ovulate cones of Pinus Banksiana 2 cm. long. Formula: 



Gum arabic 1 . g. 



Bichromate of potash . 2 g. 



Water 100. c.c. 



The mixture will not keep ; the formula is given merely to indicate 

 its composition. Make a 1 per cent solution of gum arabic in water, 

 which will keep as well as Mayer's fixative; but make the bichromate 

 solution immediately before using. Do not make the solution 

 stronger than 1 per cent; usually 0.2 per cent is strong enough. 

 Dr. Land does not measure, but simply adds enough bichromate 

 crystals to make the water pale yellow. 



