STEMS 713 



of the leaves of woody plants, associated with a definite process of 

 abscission (except rarely, as in Polygonatnm, fig. 983), but the death 

 of the stem, even more definitely than that of the leaf, is occasioned by 

 exposure to severe conditions. Only such parts survive as are in or 

 near the soil. In stem fall, as it may be called, the stem gradually rots 

 or weakens and falls to the ground and thus is comparable to leaf fall 

 in ferns or in such trees as the beech and many oaks, where death 

 ensues without the development of a well-defined absciss layer. In 

 periods of drought, transpiration becomes too great to permit the survi- 

 val of most aerial herbaceous stems, and in periods of cold also, tran- 

 spiration becomes relatively excessive in proportion to the reduced rate 

 of absorption. Aerial organs may die also as a direct result of freez- 

 ing, though many plants, especially in alpine and arctic regions, are 

 quite uninjured thereby, partly, perhaps, because of obviously pro- 

 tective structures, but more because of low water content, high osmotic 

 pressure, or other and mostly unknown features. Tropical plants, 

 on the other hand, may suffer injury or even death before the 

 temperature reaches the freezing point of water. Many stems die 

 in summer, even when the water supply is adequate; sometimes 

 such death is attributed to old age, but old age is hardly a cause 

 of death, being rather a result of certain causes, as yet imperfectly 

 understood (Part II, p. 480). 



Perennial deciduous herbs. In many perennial herbs essentially all 

 stem organs are aerial, the perennating portions being just above the 

 soil, as in Lechea, Satureja glabra (fig. 985), and Linaria canadensis; in 

 these plants lateral shoots arise at the base of the erect stem and develop 

 into short runners that remain over winter, while the erect stem dies. In 

 many runner plants, as Fragaria, Hydrocotyle (fig. 712), and Trifolium 

 repens, there are no prominent erect stems, the summer and the winter 

 conditions thus being essentially alike. An herbaceous group with more 

 numerous representatives is that in which the perennating organ is a 

 multicipital stem (as in the dock and the dandelion, fig. 995) ; here the 

 stem seems to disappear completely, the basal lateral shoots remaining 

 embryonic through the inclement season. The majority of herbs in 

 periodic climates perennate by means of underground organs, thus 

 disappearing from view during the season of inactivity; the erect stem 

 dies to the ground line or lower, and the stem is represented only by 

 the rhizome (figs. 978-981, 983), tuber (figs. 989, 990), bulb (fig. 991), 

 or corm (fig. 993). 



