66 



HUTCHINSON'S POPULAR BOTANY 



r 1 help to prevent radiation 

 at night, and thus preserve 

 the blossoms and allow the 

 fruit to set ; after all, it is 

 usually only a matter of four 

 or five degrees' fall of tem- 

 perature just at sunrise that 

 does all the damage." 



We may add that the 

 oil of Birch bark mentioned 

 above is simply a form of 

 tannin, which is one of the 

 most widely distributed of 

 secondary products. Its 

 characteristic reaction is 

 that of forming insoluble 

 compounds with gelatine, 

 solid muscular fibre, skin, 

 etc., which then acquires 

 the property of resisting 

 putrefaction, as in the tan- 

 ning of leather. Kerner 

 has pointed out that its ex- 

 tremely bitter taste protects 

 the branches, cortex, and 

 fruits from being eaten. 

 The plants which furnish 

 most of the tannin of com- 

 merce are the Oak (chiefly 

 Quercus sessifolia, infectoria, 

 and pedunculata], Hemlock 

 Spruce (Abies canadensis), 



Red Pine (Pinus contorta], and Water-smartweed (Polygonum amphibium). 

 Other by-products of metabolism and the last that we shall here speak 

 of are resins, waxes, and balsams, which naturally fall into one group. 



Young buds are often coated with a balsam (i.e. a solution of resin in an 

 ethereal oil) to protect them from cold and wet during the winter and early 

 spring. The Horse-chestnut (^fisculus hippocastanum) and Balsam Poplar 

 (Populus balsamifera) offer familiar examples of these varnished buds. 

 Again, the stems of many plants of the Clove order (Caryophyllacece) are 

 plentifully supplied with a sticky solution formed of resin and gum, which 

 effectually forbids the approach of insects to the flower along that route : 

 while resin-ducts are largely present in trees of the Terebinth and Cone- 

 bearing orders (Anacardiacece and Coniferce). The resin-producing capabilities 



Photo 6y] [E. Step. 



FIG. 95. CONE OF SABINE'S PINE (Pinus sabiniana). 



One of the finest of the Pine-cones, measuring 8 or 9 inches in 

 length. The tree is a native of California, where it grows to a 



height of 50 or 60 feet. 



