724 



ECOLOGY 



FIG. 1039. A cross section of an edge of the needle- 

 like leaf of the Austrian pine (Pinus Laricio), showing 

 the epidermis (c) with its much thickened walls, the 

 outer part (e) Jieing cutinized and the inner part (/) 

 not ; x, hypodermal sclercnchymatous tissue ; /, chlorcn^ 

 chyma" with infolded cell walls, the outermost cells (/>) 

 having these walls perpendicular to the surface, sugnest- 

 ing palisades; s, stoma with guard cells (g), subsidiary 

 cells (6), stomatal~cavify (/'), and pit (0); r, resin duct, 

 the secretory -cells (y) being surrounded by a scleren- 

 chymatous cylinder "'(#'); highly magnified. 



glucose; in addition they 

 have remarkable fila- 

 mentous nuclei. In 

 some conifers, reservoirs 

 filled with resin occur 

 in the bark, as in the 

 balsam blisters of the fir 

 (Abies balsamed). 



The role of duct se- 

 cretions. Probably the 

 contentsof resin and mu- 

 cilage ducts are chiefly 

 waste products. Since 

 such substances isually 

 cannot be excreted ex- 

 ternally, it is presi mably 

 advantageous that they 

 accumulate in reservoirs 

 outside the regions of 

 nutritive activity. Even 

 if resins and similar ex- 

 cretions are waste prod- 



ucts, they may h^,ve subsidiary advantages; for example, they may 

 preserve the wood from decay, as in the conifers, thus facilitating 

 longevity. Perhaps resins and gums are of advantage in healing wounds 

 and in checking loss of water, as in the pines and cherries, where they 

 exude copiously at the injured places. Incisions cause not only the flow 

 of resin, but also in some cases the development of accessory ducts. 



Tannins and other bark excretions. Among the commoner excretions found in 

 bark (as in the oak) are tannins, which are astringent glucosids. In Sambuc-is the 

 tannin is contained in special sacs, twenty millimeters long or thereabouts, but 

 usually the tannin-containing cells are in rows and often near the vascular tract. 

 Brown and red colors in the bark interior often are due to tannins. Similar to the 

 tannins is salicin, which occasions the bitter taste of willow bark. Most tannins 

 doubtless are waste products and eventually they are removed through the exfolia- 

 tion of the bark ; similarly, exfoliation rids trees of many other waste products that 

 accumulate in the bark, such as alkaloids, gums, resins, and calcium oxalate. 

 Tannins, because of their bitterness, may be useful incidentally in protecting from 

 animal depredations ; some tannins, known as plastic tannins, probably are of 

 value in nutrition. Tannin production appears to be especially characteristic of 



