THE PLANT IN ACTION. 255 



is ready to flow in spring, when the tree is tapped. 

 As the sap flows from the trunk, the supply is kept 

 up by the action of the roots. 



EXEKCISE LXXVIII. 

 The Reproduction of Plants. 



The processes described in the preceding ex- 

 ercises of this chapter are only concerned in the 

 growth of the plant. As they are carried on by the 

 root, stem, and leaves, these parts are known as the 

 organs of growth, or vegetative organs. But the last 

 and crowning act in the life of the plant is the pro- 

 duction of seed, and in this process the flower is the 

 portion immediately concerned. Flowers are hence 

 called the organs of reproduction. The influence of 

 pollen upon the pistil of a flower is called fertiliza- 

 tion. 



Except in rare instances, unless the ovules of a 

 plant are acted upon by pollen, seeds do not appear. 

 This is proved both by observation and experi- 

 ment. You may prevent the production of seed by 

 cutting away the stigma of the flower before the 

 ripening of the pollen. In the case of ? flowers 

 there is no seed, unless they are accessible to the 

 pollen of <$ flowers. A $ palm-tree, growing in a 

 green-house at Berlin, for twenty-four years had not 

 borne seed ; but when some pollen, sent from a dis- 

 tance by mail, was artificially supplied to the stigmas 

 of the tree, for the first time it bore fruit. Again, for 

 eighteen years it was sterile, and in the same way it 



