STEMS 



725 



xerophytes ; desert plants growing in mesophytic conditions have much less tannin 

 than in their natural habitat. In Jussiaea, tannin formation has been shown to 

 be favored by exposure to dry air and to light. 



The accumulation of waste in wood. — In many trees the heart-wood serves as a 

 reservoir of various excreta which may give it a color different from that of the sap- 

 wood, as in the red cedar (Juniperus virginiana) and in the black walnut. The 

 colored heart-wood usually is much harder than the white sap-wood (as in ma- 

 hogany and ebony), whence the significance of the name duramen. Occasionally 

 the medulla is a reservoir of excreta, as in the sumac, where it is colored yellow. 

 Doubtless the chief advantage of the accumulation of such substances in the dura- 

 men is that thus they are removed from the active tissues, though it is an important 

 subsidiary advantage that they increase the durability of the heart-wood and thus 

 promote longevity. 



7. VARIATION IN STEM FORM 



Elongation in aerial stems. — Variation in tree form. — Forest in- 

 dividuals of most trees differ widely in form from individuals grown in 

 the open, the trunks of the 

 former being tall and slender, 

 while those of the latter are 

 short and stout. Further- 

 more, trees in the open are 

 profusely branched, even 

 near the base, whereas in 

 forest individuals the 

 branches at the lower levels 

 soon die and fall to the 

 ground, leaving the tree rel- 

 atively unbranched except 

 near the top. Many herbs 

 show comparable phenom- 

 ena, isolated individuals 

 being relatively short, stout, 

 and branched, and crowded 

 individuals relatively tall, 

 slender, and unbranched; 

 in dense cultures the lower 

 leaves die much sooner than 

 on isolated individuals. The 

 death of the lower leaves in 



Figs. 1040-1042. Young plants of a prickly 

 pear cactus (Opuntia Rajinesquii), showing the on- 

 togenetic differentiation of a flattened stem from a 

 cylindrical "juvenile" stem: 1040, a very young 

 plant with a. cylindrical stem, showing the prod- 

 uct of two growth periods; 1041, an older plant 

 in which the second segment, although cylindrical, 

 is much broader than the first; 1042, a still older 

 plant in which the third segment has the flattened 

 form characteristic of "adult" individuals of the 

 genus; the small and soon deciduous leaves (/) 

 bear spines {s) in their axils. 



