WORK OF THE STEM 83 
to deposit food during winter to begin the growth of the 
following spring. But in those plants which die down to 
the ground at the beginning of winter the storage must be 
either in the roots, as has been described in Sect. 49, or in 
underground portions of the stem. 
Rootstocks, tubers, and bulbs seem to have been devel- 
oped by plants to answer as storehouses through the win- 
ter (or in some countries 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 repre- 
sents 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 alter- 
nate? 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 pota- 
toes 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 ascertain what this scale represents. 
Cut the potato across and notice the faint broken line which 
forms a sort of oval figure some distance inside the skin. 
Place the cut surface in eosin solution, allow the potato to stand 
there for many hours, and then examine, by slicing off pieces parallel 
to the cut surface, to see how far and into what portions the solution 
has penetrated. Refer to the notes on the study of the parsnip 
(Sect. 47) and see how far the behavior of the potato treated with 
eosin solution agrees with that of the parsnip so treated. 
1 Examination of a lot of potatoes will usually discover specimens with an 
inch or more of attached stem. 
