328 PHYSIOLOGY OF GROWTH AND CONFIGURATION 



up of three primary parts or organs — roots, stems and leaves; all other organs not 

 at first recognizable as roots, stems or leaves are regarded as modifications of one 

 of these three types. Thus floral parts are considered as modified leaves. Potato 

 tubers are a special kind of short, thick, underground stems, since they are 

 formed on subterranean stem-branches and not on the roots. The so-called 

 potato-eyes are dormant buds with embryonic leaves, which furnish additional 

 evidence that the tuber is really a kind of stem. This conception arose as 

 the result of simple observation and comparison, but experimental support 

 therefor was furnished by Vochting, 1 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 



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



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 

 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 that 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- 



1 Vochting, Hermann, Ueber die Bildung der Knollen. Bibliotheca botanica i 4 : n-53- 1887. 



