PLANT NUTRITION. 35 



chlorophyll. Lastly, there is the class of true parasites, 

 such as the broom-rapes ( Orobanche) and dodders ( Cus- 

 cuta), which affix themselves to living plants, and being 

 themselves destitute of chlorophyll, are unable to live, 

 except at the expense of the plants upon which they 

 grow. 



Transpiration of Water, There is a large absorption 

 of water, as has been said, by means of the root, and in 

 some cases, at any rate, there is an absorption of the 

 same fluid or vapor by means of the leaves. On the other 

 hand, there is a loss of water or watery vapor from the 

 surface, which is sometimes so profuse as to cause the 

 plant to wither and flag. We have only to place some 

 leaves in a cool tumbler, and expose them to the light, 

 to see the condensed water on the sides of the glass. 

 The quantity of watery vapor emitted in sunlight by the 

 green surfaces of plants is enormous, and it has been 

 shown experimentally that it is the chlorophyll which is 

 largely concerned in this outflow, for where that sub- 

 stance is deficient and wanting, transpiration of fluid is 

 proportionately reduced or stopped. But while bright 

 light, such as that furnished by the red and yellow ray 

 of the solar spectrum, is most efficacious in stimulating 

 the decomposition of carbonic acid, it is the blue ray 

 which specially favors transpiration of the fluid. A high 

 degree of temperature, as might be expected, favors 

 transpiration, as does also a dry state of the atmosphere. 

 The condition of the plant, its age, and other circum- 

 stances are also important agents in regulating the 

 amount of transpiration. 



Some idea of the amount of water given off may be 

 gleaned from some experiments made by Sir JohnLawes, 

 and recorded in the " Journal" of the Horticultural 

 Society for 1850, thus During one hundred and seventy- 

 two days, March 19 to September 7, the total weight of 



