ILLUSTRATIVE STUDIES 21 



this cannot be done study cross sections of onion root tips 

 that have been prepared by fixing the tips in chrom-acetic 

 fixative (page 260) and cutting the sections free-hand while 

 the tips are enclosed in elder pith (page 251), and finally stain- 

 ing in safranin and mounting in dilute glycerine (page 326). 



To secure the onion root tips boil a disk of carpet paper to 

 moisten it and kill possible fungal parasites, lay it on glass 

 or tile, place a few onions with basal ends on the moistened paper, 

 and cover with a bell-jar. When the roots are about 3 mm. 

 long cut them off and place them in Flemming's fixative (page 

 258). Proceed as directed under Cytological Methods until 

 the sections are stained in safranin-gentian violet-orange, or 

 in iron-haematoxylin (page 264), and mounted in balsam. 



Find all the parts of the cells as described in the chapter. 

 Draw younger and older cells to scale, using Fig. i as a model 

 for line-work and stippling. By means of the eyepiece microm- 

 eter determine in millimeters the diameters of the cells, nuclei, 

 and cell-walls. The plastids are so small here that they cannot 

 be identified with certainty. If the sections show nuclear and 

 cell division, as they should if the sections have been properly 

 prepared, find the different stages illustrated in Fig. 3 and 

 draw them to scale. 



3. Mount a fresh moss leaf in a drop of water and study 

 the chloroplasts with a high power. Draw to scale a single 

 cell with its contained chloroplasts. Measure the chloroplasts. 

 Possibly some of them will be found dividing by constriction. 

 With a colored pencil tint the drawing of the chloroplasts green. 



4. Make free-hand sections of the ray florets of Zinnias of 

 different colors while holding the florets in elder pith (page 

 251). Mount the sections in dilute glycerine and study them 

 quickly before the pigments that are in solution in the cell- 

 sap have time to escape. Find the yellow and orange chromo- 

 plasts in some sections and colored cell-sap in others, and some- 

 times a combination of both in the same section. Draw a few 

 cells in each case and use colored pencils to express the colors. 



5. Crush out between coverglass and slide a very small bit 



