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 with a 

 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 nave 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 



