REPRODUCTION 259 



species is always more important than its increase, and as 

 the increase of the species is only sometimes possible, such a 

 system of reproduction as will best serve the chief end has 

 been developed by every successful organism. Many or- 

 ganisms possess one means of reproduction which combines 

 these two ends, others have different means leading to the 

 two ends separately.* 



Two modes of reproduction, almost universal in their 

 occurrence among plants, can be distinguished, the sexual 

 and the non-sexual. In the former, two cells from two 

 different sources unite and thereby form a new individual. 

 In the latter, the new individual is developed from one cell 

 or from a group of cells. The difference between these two 

 would seem very clear were it not for the fact that it some- 

 times happens spontaneously in nature, and may be made 

 to happen in a considerable number of cases in the labora- 

 tory, that one of the sexual elements (cells) develops into 

 a new individual without first fusing with the other sexual 

 element (cell). The development of one sexual cell into a 

 new individual without first fusing with the other sexual cell 

 is called parthenogenesis. Parthenogenesis is, in effect, the 

 same as non-sexual reproduction. Morphologically, sexual 

 reproduction differs from non-sexual reproduction in the 

 fusion of two cells into one. Physiologically, sexual repro- 

 duction differs from non-sexual reproduction in the causes 

 which lead to it and in the results produced by it. In 

 sexual reproduction, one cell is " fertilized" by the fusion 

 with it of the other sexual element. In non-sexual reproduc- 

 tion the cells are "fertile" without this fusion. "Fertiliza- 

 tion" has been regarded as giving the stimulus needed by 

 the sexual cell for development as a new individual. Non- 

 sexual reproductive cells do not require this stimulus to 

 develop as new individuals. In natural parthenogenesis, the 

 one sexual cell, the egg-cell, also develops as a new indi- 

 vidual without the stimulus of fertilization. In partheno- 

 genesis artificially produced in the laboratory, chemical or 

 other stimuli applied to the eggs cause them to develop as 



* What these means are. may be learned from Campbell's University 

 Text Book of Botany. New York, 1902. 



