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. 



