STEMS 



31 



food of the plant is manufactured in the chlorophyll-bear- 

 ing (green) cells above ground. 



2. Tubers. — These are fleshy, underground stems. The 

 best example is common Irish potato. Although the potato, 

 ordinarily, would not be considered a stem, still if we follow 

 through its development, and examine its structure, we are 

 convinced that it is stem (Fig. 12). When we plant a 

 slice of a potato, "sprouts" are soon sent out from the 

 "eyes." These sprouts, with their nodes, and internodes, 

 and their scale leaves, are quite obviously horizontal under- 

 ground stems (rhizomes). Soon, 

 the tip of a rhizome begins to en- 

 large, and a potato is formed; 

 hence, the potato is seen to be a 

 simple enlargement of the tip of an 

 underground stem. Furthermore, 

 examination of the tuber reveals 

 the presence of a terminal bud 

 ("seed end" of the potato), and 

 lateral buds along the sides. The 

 buds are the so-called "eyes." 

 In an elongated potato, we may 

 be able to detect the spiral ar- 

 rangement of the buds. Lenticels 

 may also be observed on the corky 

 layer (skin) of the bark of the 

 potato. 



A section of a tuber reveals a stem structure. The three 

 principal parts of an ordinary stem are bark, wood and pith. 

 This is shown in a cross-section of an ordinary twig (Fig. 13). 

 In the potato, these three distinct zones are visible, as indi- 

 cated in Fig. 236. Hence, we see that the potato is in reality 

 a modified stem. 



Fig. 13. — Section of stem 

 showing a shedding leaf; also 

 bark, wood and pith as seen 

 in cross and longitudinal sec- 

 tions. (After Longyear.) 



