a 7 
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 be ? 
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 with an inch or 
more of attached stem. 
