REPRODUCTION 97 



egg of the sea urchin, can now divide and the new cells 

 can differentiate. The process of differentiation con- 

 tinues until a tiny new plant called a seed is formed. 



Fig. 59.— Diagrammatic section of a flower, showing a pollen grain sending a pollen 

 tube down to the ovule. C, calyx; Co, corolla; a, anther, and /, filament, of 

 the stamen ; O, ovary surmounted by «, style, and st, stigma; p, pollen grains, 

 some in the anther, others on the stigma; iV, ovule ; E, germ cell ; pt, pollen tube 

 penetrating the style and reaching the germ cell through an opening (the 

 micropylej into the ovule. 



Under proper conditions this is able to unfold itself 

 into a plant like the parent. 



Summary. — Reproduction may be non-sexual or 

 sexual. In non-sexual reproduction a cell, or a collec- 

 tion of cells, breaks away from the parent and develops 

 into a new organism. There is no union of cells. In 

 sexual reproduction two cells unite to form a new cell 

 which has the power to divide and differentiate. In 

 one-celled forms the uniting cells are similar and we call 

 the process conjugation. In higher forms the cells are 



