UNDERGROUND STEMS. 



The following topics are suggested to assist in the observations 

 upon underground stems and roots : 



(i) The relation of each year's growth to that of the preceding 

 year. 



(2) The modification of leaves, also their arrangement. 



(3) Definite annual growth in root-stocks. 



(4) Fluctuation in internodal growth during a year. 



(5) The form and method of branching of roots. 



I. TUBER OF JERUSALEM ARTICHOKE. {Helianthus tuberosus.) 



Each tuber has a strong terminal bud. This bud is clothed 

 with a few opposite membranous scales. They are the much 

 modified homologues of leaves. 



There are several rings around the tuber. These mark the 

 nodes. Careful observation will show that there are two modi- 

 fied leaves at each node. They appear as narrow folds meeting 

 at their extremities, thus forming a complete ring. The true 

 leaves upon the lower part of the stem of the Jerusalem Artichoke, 

 are opposite ; the upper ones are alternate. 



In the axils of the modified leaves of the tuber are axillary 

 buds, sometimes apparently wanting, sometimes developed into 

 almost independent tubers. Each pair decussates with the next 

 pair above and with the next below. Notice that the buds are 

 more strongly developed upon the upper than upon the lower 

 .side of the tuber. Secondary roots appear upon the lower side. 

 This illustrates the positive geotropism of roots, and the negative 

 geotropism of stems, even when both are below the surface of the 

 ground. 



What is the common form assumed by this tuber ? 



Ways in which underground stems differ from other stems : 



A. Entire absence of chlorophyll and hence of the green color. 



B. Much greater irregularity in the length of the internodes. 



C. Leaves usually modified into scale-like rudiments, protect- 

 ing, more or less, the axillary buds, as in the plant last studied 



