DEVELOPMENT AND REPRODUCTION 289 



the result of simple observation and comparison, but experiinental support' 

 therefor was furnished by Vochting,' who found that potato tubers may be 

 made to develop above ground, on branches that arise when the lower portion of 

 the plant is deprived of light (Fig. 165). 



Absence of light may therefore be considered as a condition favoring the for- 

 mation of tubers, but it is not an essential condition for this, for aerial tubers 

 may be obtained in light also. A leafy shoot is cut from the potato plant and all 

 buds are carefully removed from the basal portion, after which the shoot is so 

 planted in soil that there are no underground buds. Roots develop and a new 

 plant is formed but no underground branches can develop, and consequently no 

 underground tubers, on account of the absence of buds from the subterranean 

 part of the stem. As the plant grows the starch that is formed in the leaves 



Fig. 166. — Formation of aerial tubers from ordinary buds of the potato plant. (After VSchting.) 



accumulates in the ordinary buds, above the soil surface, and these develop 

 into aerial tubers (Fig. 166). These are very similar to underground tubers, 

 except that they are bright cherry red in color and have large eyes which bear 

 green leaves. Under such conditions the tubers are always formed at the base 

 of the stem but they may be produced near the tip by placing this portion in a 

 dark chamber (Fig. 167). In the latter experiment the direction of the move- 

 ment of organic materials through the stem occurs mainly in the direction oppo- 

 site to that in which it usually occurs; these substances here move from the 

 leaves below to the tubers above. 



Vochting^ showed that tuber-formation in the potato is dependent also upon 



> Vochting, Hermann, Ueber die Bildung der Knollen. Bibliothoca botanica i': ii-S3. 1887. 

 s VSchting, Hermann, Ueber die Keimung der KartoffelknoUen. Experimentelle Untersuohungen. 

 Bot. Zeitg. 60^: 87-114. 1902. 

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