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 minule 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. 



