36 THE ECOLOGY OF STEMS 



leafless evergreen herbs, such as the prickly pear (Opuntia). In all 

 cases these herbs are better protected than the corresponding ever- 

 green trees and shrubs because of their low stature. The leafless 

 herbs can endure full exposure but the majority of those that have 

 leaves grow in places where they receive some protection from fallen 

 leaves of other plants or from a covering of snow. 



(/) Deciduous Herbs.— These are perennial plants in which all 

 aerial parts die at the beginning of an unfavorable season. In some, 

 as the blood-root (Sanguinaria canadensis), only leaves are deciduous 

 but in a greater number there are aerial stems which also die down to 

 the ground each year. In connection with these latter it is proper to 

 speak of stem fall as well as leaf fall. These plants, since they live 

 through the unfavorable season as underground structures, are 

 exceedingly well protected, but they have a considerable amount of 

 construction work to do at the beginning of each growing season and, 

 of course, they cannot display their foliage as efBciently as plants 

 with perennial aerial stems. 



(g) Annual Plaiits.— Annual plants are better protected during 

 the unfavorable seasons than any other kinds because they live 

 through those seasons only as seeds. During the growing season, 

 however, these plants are practically without protection, but this is 

 not disadvantageous. The chief disadvantage of the annual habit 

 is the necessity of reconstructing the entire plant each year. 



22. Stems as Storage Organs.— Stems are nearly as important as 

 storage organs as are roots. One of the most conspicuous structural 

 characteristics of the stems of water plants is the presence of large 

 air spaces where oxj^gen and carbon dioxide accumulate in consider- 

 able quantities. On the other hand, the cacti and many other plants 

 that grow in places that are either physically or physiologically dry 

 are characterized by fleshy stems which accumulate water. Such 

 stems are spoken of as being succulent. 



Food accumulates in greater or less quantities in practically all 

 sorts of stems. It is found mostly in the parenchyma tissues of the 

 cortex, medullary rays, pith, and w^ood. This food consists largely of 

 carbohydrates, chiefly starch. In woody stems, especially, mucli of 

 the starch is changed to sugar during cold weather so that the amount 

 of sugar increases and the amount of starch decreases as the winter 

 season progresses. Underground stems obviously have the same ad- 

 vantages as roots as storage organs. 



Some families of plants are characterized by the presence of a 

 milky juice called latex. This is usually white though in some cases 



