LIVING PARTS OF THE STEM 115 
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 
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 solution 
has penetrated. Refer to the notes on the study of the parsnip 
(Sect. 56), and see how far the behavior of the potato treated with 
eosin solution agrees with that of the parsnip so treated. 
Cut a thin section at right angles to the skin, and examine witha 
high power. Moisten the section with iodine solution and examine 
again. 
If possible, secure a potato which has been sprouting in a warm 
place for a month or more (the longer the better), and look near 
the origins of the sprouts for evidences of the loss of material from 
the tuber. 
EXPERIMENT XXIII 
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 reweigh. What does the result show in 
regard to the use of the corky layer of the skin? 
124. Morphology of the Potato. —It is evident that in 
the potato we have to do with a very greatly modified 
form of stem. The corky layer of the bark is well repre- 
sented, and the loose cellular layer beneath is very greatly 
developed; wood is almost lacking, being present only in 
the very narrow ring which was stained by the red ink, 
but the pith is greatly developed and constitutes the prin- 
cipal bulk of the tuber. All this is readily understood if 
we consider that the tuber, buried in and supported by 
the earth, does not need the kinds of tissue which give 
