CHAPTER XL 

 WORK OF THE LEAVES. 



152. 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, 

 or energesis, the energizing of the plant, which results in waste; 

 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. 



153. The loss of water by leaves. That the loss of water 

 by plants is chiefly through leaves can be shown by an interesting 



Fig. 80. 



After a few hours' drops of water have accumulated on the inside of the 

 jar covering the leaves. 



experiment as follows. A few fresh leaves are cut from a plant, 

 and the ends of the petioles are sealed by dipping them in paraffin, 

 These are now placed under a dry bell jar or inverted fruit jar, 



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