STEMS 737 



trolled in the main by definite external factors rather than by an 

 influence residing in the main shoot. Elongation in spring may be 

 associated with the vigorous movement of structural materials at that 

 season, and subsequent dwarfing may be associated with a reduced 

 movement, perhaps supplemented by increased desiccation. The 

 amount of branch elongation varies with the season and is reciprocal to 

 the width of the annual ring (p. 691), maximum elongation occurring in 

 moist and minimum elongation in dry seasons. Differences between 

 twigs of different species (e.g. the slender elongated twigs of willows, as 

 compared with the stout twigs of the sumacs) appear inherent rather than 

 related to external factors. 



Additional periodic phenomena. Periodicity is exhibited in the daily growth of 

 stems, elongation being greater by night than by day, as is illustrated by the bam- 

 boo, of whose extraordinary growth about two thirds occurs by night. The factors 

 here involved are complex, but may be associated in part with the absence of light 

 (directly or indirectly or both) and with lessened transpiration. Notable periodicity 

 is exhibited by biennial rosette plants (fig. 1036), in which, possibly because of insuffi- 

 cient structural material, there is no stem development during the first season, though 

 a vigorous erect stem rises in the second season. Alternating elongated and dwarfed 

 shoots also are illustrated by the summer stems and winter buds of Utricularia 

 (p. 678), the phenomena and perhaps the causative factors recalling the situation in 

 tree branches. 



Inherent rhythm. While the plants of uniform climates are evergreen, they 

 are not necessarily continuous growers, though there rarely occurs such definite 

 periodicity as in periodic climates. Some trees, as the coconut and the papaw 

 (Carica), are essentially ever growers, developing both vegetative and reproductive 

 organs on the same shoot in unbroken continuity. Probably most trees of the rain 

 forest exhibit what might be termed spasmodicity in contrast to periodicity, the vary- 

 ing branches appearing independent of one another and of external conditions; one 

 branch elongates and puts forth new leaves, while another is blooming, another fruit- 

 ing, and still another is quiescent. When the grape is taken from a temperate region 

 to the rain forest, it develops this spasmodic habit. Such plants, then, are neither 

 uniform evergrowers nor are they uniformly rhythmic, but each branch appears to 

 exhibit a rhythm of its own. It is doubtful, however, if this or any other plant 

 rhythm is entirely unrelated to rhythmic external conditions. 



The origin of trees. The most distinctive features of trees are per- 

 ennial elongation, perennial diametral increase, perennial lignification, 

 and permanence of tissues. Elongation has been seen to be stimu- 

 lated by conditions that favor absorption or impede desiccation, while 

 lignification and diametral increase are favored more by xerophytic 

 conditions. Maximum elongation, on the whole, occurs in the tropical 

 rain forest, where trees are predominantly tall and slender ? though the 



