32 HEREDITY AND EVOLUTION IN PLANTS 



may be severed from the parent plant. This is called 

 layering. The production of new individuals by the arti- 

 ficial methods of the gardener is called propagation; but 

 between these methods and multiplication by special 

 bodies, given off spontaneously by the plant, no hard and 

 fast line can be drawn. Some plants, for example, be- 

 come layered without the gardener's assistance; other 

 plants (as the willow), by self-pruning, spontaneously 

 give off branches from which new plants may develop; 

 while, on the other hand, the gardener may cut a tuber, 

 such as the "potato " into a number of pieces, from each of 

 which a new plant will develop. In this practice artificial 

 propagation and vegetative multiplication are combined. 



23. Reproduction by Spores. — The essential fact about 

 a spore is that it is an individual cell or small group of 

 cells, produced primarily for reproductive purposes, 

 given off by the plant, and capable by itself of producing 

 a new individual. The essence of all reproduction is the 

 separation of the reproducing cell or body from the parent 

 plant. If a bud or a bulb remains attached to the plant 

 that formed it, it produces only a branch or other organ, 

 but not a new individual. So, also, a spore must be sepa- 

 rated from the parent plant in order to reproduce the 

 latter. In many cases spores may germinate before they 

 are set free, but the separation must come sooner or later. 



24. Sexual Reproduction. — In marked contrast to 

 reproduction by spores, is the reproduction by means of 

 sperms and eggs, involving cell- and nuclear-fusions, known 

 as fertilization. Eggs and sperms are called gametes,^ 

 the egg being the female gamete, the sperm the male 

 gamete. The diploid cell, resulting from the union of two 

 gametes, is called a zygote, and this term is often extended 



' From the Greek word, yijuos {gamos), meaning marriage. 



