UNDERGROUND STEMS 159 



and about a foot apart. From the eyes new plants 

 develop. Potatoes do well in soil which may not be good 

 for grain crops. A loose soil is better for them than a 

 hard, compact one ; it offers less resistance to the enlarge- 

 ment of the tubers. After the shoots have come up, the 

 soil is heaped up into hills about the base of the plant. 

 This stimulates the formation of tuber-bearing branches. 



Very likely you have noticed potatoes sprouting in the 

 cellar. Long, pale shoots grow from the eyes. They seem 

 to be seeking the light. If they find it, they soon turn 

 green. This is on account of the manufacture of chloro- 

 phyll. Evidently the plant does not manufacture chloro- 

 phyll in the dark. 



Another tuber important for food is the Jerusalem 

 artichoke. The plant is a sort of sunflower. In this tuber 

 the nodes and the buds are much more prominently de- 

 veloped than in the potato. 



C. Bulbs. — An onion is the most familiar kind of bulb. 

 Lily, tuhp, and hyacinth bulbs are also well known. A 

 bulb is a modified shoot in which food is stored, the leaves 

 being more prominently developed than the stem. In 

 onion and tulip bulbs the leaves are so broad that they com- 

 pletely cover those within. In lily bulbs the leaves over- 

 lap, but do not completely cover each other. (See Figure 



55-) 



Tuhp beds are usually made by planting the bulbs 

 rather late in the fall. The cold of winter does not injure 

 them, and they prepare for growth in early spring. It is 

 well to cover the beds with straw and manure. Such a 

 covering is called a mulch. The mulch prevents sudden 

 changes in temperature in the ground beneath it. It also 



