134 ROOTS AND UNDERGROUND STEMS 



tubers of the Madeira vine also make good objects for 

 study.) Notice the arrangement of the eyes, or buds; is 

 it alternate or opposite ? How many ranked ? Make a 

 sketch of the potato as it appears on the outside. Make 

 a similar sketch of the sweet potato, and compare the 

 two. Is there any scale below the eye in the sweet 

 potato ? Do the eyes occur in any regular order .'' 



191. Make a cross section of each, and sketch them. 

 Notice the thin dark ring that runs around the inside of the 

 potato at some distance from the circumference. Label 

 this vascular tissue ; the loose porous layer between it and 

 the skin, cortex; the central portion within the vascular ring, 

 pith ; and the outer skin, epidermis. See if you can find 

 cowesponding parts in the sweet potato, and label them. 



Put one of the cut ends of each in red ink (this should 

 have been attended to before the recitation), let them stand 

 four to five hours, then make sections parallel to the cut 

 surface till you reach the point where the red ink has pene- 

 trated ; what difference do you notice .-" Which has the 

 thicker cortex .? Compare the behavior of the potato with 

 that of the turnip treated with red ink in Section 179. 

 What would you infer from this as to the office of the woody 

 tissue } What is the office of the epidermis .'' If you are 

 in doubt, peel a tuber and weigh it. At the same time 

 weigh one of about the same size from which the skin has 

 not been removed, and put the two side by side in a dry 

 place. At the end of three or four days weigh them again 

 and see which has lost the most. 



We have learned that roots are not divided into nodes, 

 that they never bear leaves, that they branch quite irreg- 

 ularly, and that they sometimes bear adventitious buds. 

 Now can you state some of the reasons why the potato 

 is regarded as a stem and the sweet potato as a root ? 



192, Storage of Nourishment. — The object of both is 

 the same, the storage of nourishment. Drop a little iodine 

 on each and see what this nourishment consists of. 

 Which contains the more starch .■' 



