CHAPTER XIII 

 LEAVES — FUNCTION OR WORK 



We have discussed (in Chap. VIII) the worlc or function 

 of roots and also (in Chap. X) the function of stems. 

 We are now ready to complete the view of the main vital 

 activities of plants by considering the function of the 

 green parts (leaves and young shoots). 



Sources of Food. — The ordinary green plant has but two 

 sources from ivhicJi to seaire food, — the air and the soil. 

 When a plant is thoroughly dried in an oven, the water 

 passes off ; this water came from tlie soil. The remaining 

 part is called the dry substance or dry matter. If the dry 

 matter is burned in an ordinary fire, only the ash remains; 

 tliis ash came from the soil. The part that passed off as 

 gas in the burning contained the elevients that cam.c from 

 the air ; it also contained some of those that came from 

 the soil — all those (as nitrogen, hydrogen, chlorine) that 

 are transformed into gases by the heat of a common fire. 

 The part that comes from the soil (the ash) is small in 

 amount, being considerably less than lO per cent and 

 sometimes less than i per cent. Water is the most 

 abundant single constituent or substance of plants. In a 

 corn plant of the roasting-ear stage, about 80 per cent of 

 the substance is water. A fresh turnip is over 90 per 

 cent water. Fresh wood of the apple tree contains about 

 45 per cent of water. 



Carbon. — Carbon enters abundantly into the composition 

 of all plants. Note what happens when a plant is burned 



