112 CONDENSED TYPE OF STEMS 



they are infested with fungi. This checks their elongation and 

 possibly accounts for the change in their mode of growth, which 

 is now characterized by the enlargement of the infested region 

 and the accumulation of starch. It is well to remember that the 

 organs of many of these plants are exceedingly plastic and con- 

 trolled to a remarkable degree by conditions. As a single instance 

 of this fact note the leafy stems originating from the "eyes" of 

 the potato are inhibited in their elongation by strong light and 

 dry air. A potato grown on a moist surface in intense light and 

 dry air develops short cactus like stems; but grown in the dark 

 and in moist air they elongate vigorously after which their 

 nature changes, for if now exposed to light they develop as the 

 ordinary leafy stem. This is one reason for planting tubers 

 several inches in the soil. 



A great variety of plants develop only very short stems that 

 are more or less buried in the ground. This type of stem modi- 

 fication has many of the advantages of the rhizome but not its 

 power to spread through the soil and so spread the plant. The 

 vital parts of such plants are only slightly exposed and conse- 

 quently suffer little from grazing animals or other sources of 

 injury, as in plantains, dandelions, etc. Frequently these short 

 stems are associated with an abundance of foods that are stored 

 in roots or other organs. Such plants can quickly send up and 

 mature their flower stalks or leafy stems and thus avoid un- 

 favorable conditions, as drought, competition with larger vege- 

 tation that will appear later, etc. These facts account for the 

 common occurrence of this type of stem in arid regions where 

 there are short rainy seasons. This habit has been made very 

 conspicuous by cultivation in many plants, as the carrot, turnip, 

 radish, and beet. But in nature the very short, almost flat stems 

 and fleshy roots are also of common occurrence, e. g., the wild 

 carrot, wood betony, dandelion, etc. In many cases the short 

 stem itself is the storage organ and consequently it becomes en- 

 larged and fleshy, as in the Jack-in-the-pulpit, garden crocus, 

 spring beauty, etc. (Fig. 73, A). These short erect stems are 

 termed corms and are suggestive of the tuber. In other plants 

 the bases of leaves attached to the short stems function for the 



