l8o STRUCTURE AND LIFE HISTORIES 
ficial methods of the gardener is called propagation; but 
between these methods and the 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. 
163. 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. No 
hard and fast line can be drawn between spores and gemmae. 
164. 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, 1 
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 
1 From the Greek word, 74/40$ (games'), meaning marriage. 
