76 THE NATURE AND WORK OF PLANTS 



specimens standing out in exposed places are twisted 

 so that they point nearly toward the north or south 

 — a fact which has given the plant its common name. 

 The effect of this arrangement is to present the edge 

 of the blades to the noonday sun, and the direct rays 

 may strike them only in mid forenoon and mid after- 

 noon, thus avoiding the fiercer rays of the noonday 

 sun. Only a dozen species show this interesting 

 method of avoiding damage to the leaves. 



95. Some plants have lost the poiver of manufac- 

 turing chloroj^hyl. — All species which take carbon 

 dioxide from the air do so by means of the help of 

 chlorophyl, and when they live in a place where 

 they may get their food already formed, the green 

 color is not needed. It is an invariable rule with 

 all living things, that as soon as an organ or a struc- 

 ture or a substance becomes less useful, the succeed- 

 ing generations of the plant do not perfect the useless 

 thing. Hence the species which have become able 

 to take up their food already formed have lost 

 their green color. The bacteria have all done so, 

 as well as the mushrooms, and moulds, and their 

 relatives. A few flowering species have undergone 

 similar changes. If a plant gets its food from the 



