LIVING PARTS OF THE STEM. 73 



countries where there is one, through the dry season)^for the 

 reserve materials which the plant has accumulated during the 

 growing season. The commonest tuber is the potato, and 

 this fact and the points of interest which it represents make 

 it especially desirable to use for a study of the underground 

 stem in a form most highly specialized for the storage of 

 starch and other valuable products. 



98. A Typical Tuber ; the Potato. — Sketch the general outline of a 

 potato, showing the attachment to the stem from which it grew.^ 



Note the distribution of the "eyes," — are they opposite or alternate ? 

 Examine them closely with the magnifying glass and then with the lowest 

 power of the microscope. What do they appear to he ? 



If the potato is a stem it may branch, — look over a lot of potatoes to 

 try to find a branching specimen. If such a one is secured, sketch it. 



Note the little scale overhanging the edge of the eye, and see if you 

 can make out what this scale represents. 



Cut the potato across, and notice the faint line which forms a sort of 

 oval figure some distance inside the skin. 



Place the cut surface in red ink, allow the potato to stand so for many- 

 hours, and then examine, by slicing off pieces parallel to the cut surface, 

 to see how far and into what portions the red ink has penetrated. Refer 

 to the notes on the study of the parsnip (§ 45), and see how far the 

 behavior of the potato treated with red ink agrees with that of the 

 parsnip so treated. 



Cut a thin section at right angles to the skin, and examine with a high 

 power. Moisten the section with iodine solution and examine again. 



Make a cross-section and a lengthwise section through the stained ring 

 from the piece left standing in red ink, and examine first with a low, 

 then with a high power. 



If possible secure a potato which has been sprouting in a warm place 

 for a month or more (the longer the better), and look for evidences of 

 the loss of material from the tuber. 



99. Experiment 20. Use of the Corky Layer. — Carefully weigh 

 a potato, then pare another larger one and cut portions from it until its 

 weight is made approximately equal to that of the first one. Expose 

 both freely to the air for some days and re-weigh. What does the result 

 show in regard to the use of the corky layer of the skin ? 



1 Examination of a lot of potatoes will usually discover specimens witli an inch or 

 more of attached stem. 



