74 



PLANT RELATIONS. 



Fig. 64. An araucarian pine, showing the 

 central shaft, and the regular clusters of 

 branches spreading in every direction and 

 bearing numerous small leaves. The low- 

 ermost branches extend downwards and 

 are the largest, while those above become 

 more horizontal and smaller. These dif- 

 ferences in the size and direction of the 

 branches secure the largest light expo- 

 sure. 



are of this character; 

 and as the main pur- 

 pose is food storage 

 the most favorable 

 position is a subter- 

 ranean one (see Fig. 

 66). Sometimes such 

 scale leaves become 

 very broad and not 

 merely overlap but en- 

 wrap one another, as 

 in the case of the 

 onion. 



51. The tuber type. 

 — The ordinary potato 

 may be taken as an il- 

 lustration (see Fig. 

 67) . The minute scale 

 leaves, to be found at 

 the "eyes" of the 

 potato, do not overlap, 

 which means that the 

 stem joints are farther 

 apart than in the bud 

 type. The whole form 

 of the stem results 

 from its use as a place 

 of food storage, and 

 hence such stems are 

 generally subterra- 

 nean. Food storage, 

 subterranean position, 

 and reduced scale 

 leaves are facts which 

 seem to follow each 

 other naturally. 



