184 APPLIED BIOLOGY 



important reservoirs of water needed in times of drought. 

 The absence of leaves gives the advantage of greatly reduced 

 surface exposed to evaporation. 



178. Stems Adapted as Leaves. In extremely dry 

 countries some plants have formed the habit of producing 

 very small leaves or none at all, in order that there may be as 

 little loss of water as possible. An extreme case of this kind 

 is seen in cacti, described in the preceding section. Our 

 garden asparagus is by nature adapted to very dry con- 

 ditions. Its only leaves are the scales which appear on the 

 young edible shoot early in the spring, and the mass of slen- 

 der green structures which develop later in the summer are 

 branches. In the greenhouse plant commonly called " smi- 

 lax," the flattened leaf-like structures are stems. This is 

 evident from their position in the axils of the very small 

 scaly leaves. Such modified stems are called cladophylls, 

 meaning branch-leaves. 



That this modification and reduction of leaf surface is an 

 adaptation to environment brought about by external in- 

 fluences is proved by the fact that certain plants which 

 produce leaves in regions of moderate water-supply become 

 very much like spiny cacti when grown in desert conditions. 



179. Economic Value of Wood Structure. The useful- 

 ness and consequent monetary value of lumber depends upon 

 the nature of the various structures seen in the sections of 

 stems ( 163). For many purposes hardness and strength 

 combined are desired (e.g., white oak for wagon-axles, hickory 

 for ax-handles and wheel-spokes). Sometimes a light, 

 elastic, straight-grained wood is wanted ; and so ash is most 

 common for the long handles of hoes, shovels, pitch-forks, 

 etc. Woods which resist decay because they contain resins, 

 oils, or other protecting substances are needed for fence-posts 

 and telephone poles ; and for such purposes cedar, chestnut, 

 oak, mulberry, and black locust are usually preferred. White 

 and red cedar, cypress, and redwood make the best shingles 



