22 THE PLANT CELL 



of the pulp of a tomato or of climbing bitter-sweet berries, 

 using the juice of the fruit as a mounting medium. Draw and 

 measure chromoplasts of different forms and colors. Tint the 

 drawings as near the natural colors as possible. 



6. To see in a comprehensive way that cell-walls may be 

 of different kinds mount thin cross sections of old stem of Aris- 

 tolochia (Fig. 24), or of some other woody stem, in different 

 reagents for differentiating cell-walls. Examine a section first 

 in water to note the natural colors of the walls. Filter away the 

 water and add a drop of chloroiodide of zinc. After a w r hile 

 the cell-walls of some of the tissues will be colored yellow and 

 others purple. The purple walls are cellulose and the yellow 

 are lignified, suberized or cutinized. 



Mount another section in aniline sulphate and only the lig- 

 nified walls will be colored yellow while the others will be left 

 unstained. Mount a third section in phloroglucin and the 

 lignified walls will be colored pink while all others will be un- 

 changed. Finally sections left for several hours in a tincture 

 of alcannin will have the cutinized and suberized wall alone 

 stained pink. See under these reagents in Chapter XVI. 



7. Mount a bit of Nitella in a drop of water and study the flow 

 of the cytoplasm. With a micrometer eyepiece and a metronome 

 to tick off seconds determine the rate of flow per second. 



Remove a filament from a young bud of Tradescantia Vir- 

 ginica and mount it in a drop of water. Study the circulation 

 of the cytoplasm in one of the cells of a hair growing from the 

 filament. How does it differ from Nitella as to the manner 

 and rate of flow? Draw a cell, using Fig. 9 as a pattern for 

 line-work and stippling. 



8. Mount in water some of the mealy green scum sometimes 

 found at the surface of stagnant ponds. If it is found composed 

 of motile green bodies with a red eyespot, these individuals are 

 Euglena viridis. Note the beating back and forth of the color- 

 less flagellum at the eyespot end, and show by drawings the 

 different forms which a single Euglena is found to assume 

 within a short time. 



