84 ESSENTIALS 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. 3.5) 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 section from 

 the interior of the bulb, examine with a moderate power of tha 

 microscope, and note the thin-walled cells of which it is composed. 



