CHAPTER XII. 

 WORK OF THE LEAVES. 



156. Work performed by leaves. There are several kinds 

 of work performed by the leaves which we will study as follows: 

 First, Transpiration, or the giving off of water; second, Photo- 

 synthesis, or the making of sugar and starch; third, Respiration, 

 the energizing of the plant; fourth, Digestion, the preparation 

 and transformation of foods; and fifth, Assimilation, or the 

 making of new living matter and the repair of waste. Several 

 of these functions are carried on by all parts of the plant, but 

 preeminently by the leaves, except in those plants so modified 

 that some other plant part has taken on the work of the leaf, as 

 in the cacti, phylloclades, etc. 



I. TRANSPIRATION. 



157. The loss of water by leaves. The loss of water from 

 leaves* can be shown by an interesting experiment as follows: 



Fig. 83. 



After a few hours, drops of water have accumulated on the inside of the jar covering the 

 leaves. 



A few fresh leaves are cut from a plant, and the ends of the peti- 

 oles are sealed by dipping them in paraffin. These are now 

 * In general the loss of water by plants is in proportion to the exposed 

 surface, so that the greater loss is through the leaves. 



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