How Substances are Transported and Removed 223 



wood long after its center has rotted away. It is true, this prin- 

 ciple would require for greatest efficiency, that the fibers should 

 lie on the very outside, as indeed they do in some cases ; but such 

 an arrangement would prevent all access of light and therefore 

 the use of the surface for spreading of chlorophyll. It is easy to 

 understand how the plant could find it advantageous to sacrifice 

 a trifle of effectiveness in the strengthening system for the sake 

 of the marked advantage of spreading more chlorophyll; and in 

 this arrangement we see one of those innumerable compromises 

 with which plants, like mankind, are accustomed to meet the con- 

 flicting problems of existence. 



Such is the primary or ground structure of stems, as typically 

 displayed in their earlier stages, and up to the time when they 

 cease to be flexible, green and soft. Then they begin to undergo 

 remarkable changes, connected adaptively with their continuous 

 growth into trees ; but these we can better postpone to our chapter 

 on Growth, where the reader will find them fully described. 



It will interest the reader to know that the principal theme of 

 this chapter, — the transfer and transpiration of water, — will al- 

 ways be associated in the minds of plant physiologists with the 

 foundation of their science; for to it, of all the phases of plant 

 physiology, was first applied that exact scientific method of 

 measurement which is the only sure means for advancing natural 

 knowledge. Its founder was Stephen Hales, whose book Vegetable 

 Statics, though published in 1727, might have been written- 

 yesterday so far as its spirit is concerned. He will always be 

 considered the father of this science, and his book one of the 

 greatest of botanical classics. 



