50 LABORATORY SrUBIES OF THE STEM 



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

 from the original attitude of the stem. 



Exercise XXII. Special Uses and Forms of Stems 



Creeping or underground stem. — Study the rhizome. Look for 

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

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

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

 food in store ? 



Draw what is needed to illustrate your notes. 



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

 base of the tuber. By base is meant the end by which the Potato was 

 originally attached to the Potato plant. 



Holding the tuber right end up, examine it. With the lens look 

 at several minute prominences within the depression of each eye. 

 These are buds. 



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

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

 ing a bud) ? Test the pulp with iodine. Morphologically, what is 

 the tuber? What is the proof? What is its purpose in the life history 

 of the potato plant ? 



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

 rudimentary buds ( x 3-4). 



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

 close-set, thickish leaves. 



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

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

 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 stem of Houseleek with the tuber of Potato in all 

 respects, — as to organs present, the comparative development of these 

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



Draw the longitudinal section of the head. 



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

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

 What is it? Follow up the successive subdivisions 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 morphology ? Note the 

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

 Asparagus ? 



