50 LABORATnRY STCDIES OF THE STEM 



note tlie time. Observe the scape again later, looking for a change 

 from the original attitude of the stem. 



Exercise XXII. SrECiAL U.ses and Forms of Stems 



Creeping or underground stem. — Studj- the rhizome. Look for 

 stem, leaf, and root. AVhich are present? What are the marks show- 

 ing the true nature of stem, if that is present? What is the distribu- 

 tion of the roots, if piresent? If thickened, does the rootstock contain 

 food in store ? 



Draw wliat is needed to ilhistrate your notes. 



Tuber of Potato. — First, try to distinguish between tlie tip and the 

 base of the tubei'. liy base is meant the end by wliicli the Potato was 

 originally attaclied to the Potato plant. 



Holding the tuber light end up, examine it. Witli tlie lens look 

 at several minute prominences Avithin the depression of each eye. 

 These are Ijuds. 



Below is a ridge, and frequently at its middle point may be seen 

 a small, erect scale. What is the morpliology of this scale (subtend- 

 ing a tiud) ? Test the pulp witli iodine. ^lorphologically, what is 

 tlie tuber? A\'liat is the piroof ? AN'liat is its purpiose in the life history 

 of the potato plant ? 



Draw au enlarged view of the eye, showing ridge, scale, and 

 nidinieiuary Ijuds (x :!-f). 



Houseleek. — (Optional.) Examine: (1) The green lieads, with 

 close-set, thickisli leaves. 



{-) The dull-colored, rootlike parts connecting them. Precisely 

 whence do the latter spring? In what do they end, and how? Crrt 

 away leaves enough to determine these questions clearly. Have they 

 any scars, scales, or appendages ? What is their morphology ? Proof ? 

 Cut a longitudinal section of one of the heads. Xote the sudden 

 enlargement of the axis at the point where the leaves begin to be 

 crowded. Apply dilute iodine. 



Compare the steiu of Ilonseleek with the tuber of Potato in all 

 respects, — as to organs jireseiit, the comparative development of the.se 

 organs, the purpose of tlie whole, and any other points. 



Di-aw the longitudinal section of the head. 



Asparagus. — Select a sprig which branches several times. At the 

 Ijase of every branch at least one small, scalelike structure is found. 

 What is it? Follow up the successive subilivisions of one of the 

 branches, arriving finally at the smallest members of the ramification. 

 At each dividing note a similar scale. Is it found at the foot of the 

 needlelike "leaves"? If so, what is their morphologv? Xote the 

 color of .ill parts of the plant. What is the function of stem in 

 Asparagu> ? 



