STEMS 



709 



In a number of trees the bark exfoliates in definite layers 

 (fig. 1034), the separation being in a zone of weakness, known 

 as the separation layer, which is composed of loose and weak 

 cells that alternate with the denser and stronger cork layers. 

 In trees with scaly bark the cork layers separate into patches 

 or arcs (as in the sycamore, cherry, and pine), while in plants 

 with ringed bark, the cork layers form concentric cylinders 

 and the bark shreds or slivers off (as in the grape and arbor 

 vitae). In trees with shaggy bark the exfoliating masses are 

 elongated, and in the birches the bark exfoliates in thin, 

 papery layers. In some trees the bark is supplemented in 

 its protective role by dead-leaf bases (as in Yucca, p. 588). 



The protective significance of different stem 

 habits. Introductory remarks. The reproductive 

 activity of stems appears to be exhibited somewhat 

 equally in all situations, but foliage display is much 

 more prominent in mesophytic than in xerophytic 

 habitats, probably because the excessive transpiration 

 in the latter makes it impossible for plants to de- 

 velop greatly elongated stems; nor could plants 

 with abundant foliage resist desiccation if such 

 development were possible. 



Tropical evergreen trees. Only in the tropical 

 rain forest is unrestricted foliage display observed, 

 for there alone because of the lack of drought or 

 cold is continued activity possible without protective 

 structures or behavior. Daily synthesis and never- 

 failing moisture combine to produce the most luxuri- 

 ant vegetation (fig. 846). An excellent example of 

 the well-nigh perfect growth conditions of the rain 

 forest is seen in the bamboo, whose stems sometimes grow as much as 

 eight meters in a single month, or at a rate of more than twenty-five 

 centimeters per day. However, the very excellence of tropical condi- 

 tions causes such a superabundance of vegetation that only an occa- 

 sional seed of any given species can fall in a place suitable for germi- 

 nation, and that only a few of the germinating plants can ever reach 

 maturity. 



Sclerophyllous evergreen trees. Except in the rain forest, most trees 

 are either deciduous or sclerophyllous (i.e. having stiff evergreen leaves), 

 and must endure inclement seasons, characterized either by drought or 



FIG. 1034. A 



portion of a stem 

 of the nine-bark 

 (Physocar pus opuli- 

 folius), showing the 

 shredding of the 

 bark into several 

 thin exfoliating 

 layers. 



