84 ELEMENTS OF BOTANY 
Cut a thin section at right angles to the skin and examine with 
a high power. Moisten the section with iodine solution and exam- 
ine 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 XII 
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? 
99. 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 represented, 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 principal 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 strength, but only those which 
are well adapted to store the requisite amount of food. 
100. Structure of a Bulb; the Onion. — Examine the external 
appearance of the onion and observe the thin membranaceous skin 
which covers it. This skin consists of the broad sheathing bases of 
the outer leaves which grew on the onion plant during the summer. 
Remove these and notice the thick scales (also formed from bases of 
leaves as shown in Fig. 35) which make up the substance of the bulb. 
Make a transverse section of the onion at about the middle and 
sketch the rings of which it is composed. Cut a thin seetion from 
the interior of the bulb, examine with a moderate power of the 
microscope, and note the thin-walled cells of which it is composed. 
