THE CELL AND THE CELL THEORY 



57 



Fertilization. The process of fertilization was first observed 

 in Spirogyra, one of the lower water plants, in 1879, and in the 

 higher plants by Strasburger in 1884. 



The essential act of this fertilizing process was found to con- 

 sist in the union of two cells, male and female, termed gametes or, 

 more frequently, sperm cell and egg cell (Fig. 29). Later studies 

 seem to indicate that the union of the male and female nuclei is 

 the most important if not the only essential part of the process. 



Embryo 



Fertilization Zygote Cell division Cell di/erentiatioj^ 

 Gametes 



Organ formation 



FIG. 29. A diagram illustrating the main cell processes occurring during the 

 development of a plant 



Consult the text for further discussion 



The two gamete cells, when united in fertilization, form a 

 new cell, the zygote cell, which is a double cell in the sense that 

 it is composed of two cells from two distinct individual plants 

 or from separate portions of one plant. This double nature of 

 the zygote cell will be found, in our later studies in reproduc- 

 tion, to be of great significance in heredity, since it contains the 

 fundaments of two sets of characters, one set brought in by the 

 male gamete and one by the female gamete. 



Embryology. The development of a higher plant from the 

 zygote was first completely worked out by Hanstein in 1869. 

 The development process of the higher plant, like that of a 

 higher animal, was found to embrace three important stages, or 

 phases, each phase including processes of cell division and 

 growth. The first stage, or phase, is concerned with the division 

 of the single-celled zygote into many cells, thus giving rise to 



