176 PHYSIOLOGICAL BOTANY. 



How the elaborated sap passes back and even 

 downward through the cells and vessels that are at 

 the same time employed conveying the crude watery 

 fluids up from the root is not understood. We are not 

 acquainted with any physical or chemical force which 

 causes the crude sap to creep through the cells and 

 ducts of the trunks and branches of great trees, hun- 

 dreds of feet in height ; nor is the transfusion of the 

 prepared fluids and cell materials to every part of the 

 plant's structure where food is required less difficult 

 to explain. 



In fact, observation and experiment have thus far 

 failed to account for these mysterious movements. 



CHAPTER VI. 



FERTILIZATION. 



433. The higher plants produce seeds, each of 

 which contains an embryo of a new plant. The seed 

 has already been defined as the ripened ovule or as 

 the fertilized and mature ovule. The fertilization of 

 the ovule is accomplished by the mingling of the 

 protoplasm of the pollen cell with the protoplasm of 

 the ovule, which is brought about in the following 

 manner : 



434. Process of Fertilization. The ripened anther 

 opens and discharges its pollen grains, some of which, 

 by the action of the wind or the aid of insects, reach 

 the stigma ; when one has secured a lodgment, influ- 

 enced by the moist surface of the stigma, it germi- 

 nates, sends down through the tube of the style a 

 tube as the radicle of the seed penetrates the earth 



