Drccnihi'i', 1912 
'IIII'; ORCHID WORLD. 
59 
When the factor is aUogether absent, the 
plant or animal may be called zerozygous, or 
wanting, m regard to that factor. 
Previous to fertilisation the germ-cells 
divide and segregation takes place in regard 
to each factor. 
Each cell with a double dose of the factor 
becomes two fertilising cells, or gametes, each 
with a single dose of the factor. 
Each cell with a single dose of the factor 
becomes two fertilising cells, or gametes, one 
of which has a single dose of the factor and 
the other has none. 
Fertilisation consists m the union of two 
fertilising cells, or gametes, a pollen or sperm- 
cell from the male uniting with an egg-cell 
from the female. 
If both paternal and maternal gametes 
have a single dose of the factor, a z}gote will 
be formed with a double dose of the factor, 
and the plant or animal will be homozygous, 
or pure, m regard to that factor. 
If, however, one of the gametes has a single 
dose of the factor and the other has none, a 
zygote will be formed with a single dose of 
the factor, and the plant or animal will be 
lieterozygous, or impure, m regard to that 
factor. 
If neither of the gametes has the factor at 
all, it will, of course, be altogether absent 
from the zygote, and the plant will be zerozy- 
gous, or wanting, m regard to that factor. 
Practical Consequences. 
The practical consequences of these first 
principles of genetics are that, as regards any 
one heritable character represented by a 
factor, there are three distinct kinds of indi- 
vidual plants: (i) homozygous, or pure; (2) 
heterozygous, or impure ; and (3) zerozygous, 
or wanting. Each of these three kinds of 
individuals will give a different result when 
bred from. With regard to outward appear- 
ances, No. 3 will, of course, be easily distin- 
guishable from Nos. i and 2, representing, as 
it usually does, the hypostatic or recessive 
form from which the epistatic or dominant 
character is quite absent. 
If, as sometimes happens, a single dose of 
the dominant factor produces the same effect 
111 the zygote as a double dose, we get the 
])henomenon of complete dominance, and 
m such cases Nos. i and 2 are indistin- 
guishable m outward appearance, and their 
homozygous or heterozygous nature can only 
be determined by breeding from them. 
If, however, as often happens m Orchids, a 
single dose of the dominant factor produces a 
different effect in the zygote from a double 
dose, we get the phenomenon of incomplete 
dominance, in such cases Nos. i and 2 are 
distinguishable m outward appearance, and 
their homozygous or heterozygous nature is 
determined at sight without breeding from 
them. 
The apparent blending of characters 
observed m many Orchid hybrids is no doubt 
due to the effect of the single dose of one 
factor reacting on the effect of a single dose 
of another factor, resulting m a mosaic. 
W^hen more than one heritable character is 
involved m the breeding problem, as usually 
happens in Orchids, the different kinds of 
individual plants become exceedingly numer- 
ous. One plant, for instance, may be 
homozygous for one character, heterozygous 
for another, and zerozygous for a third 
character, and so on. Each individual plant, 
however, has a definite germinal or gametic 
constitution, which can be ascertained by 
cross-breeding it with other plants. In this 
way a factorial analysis of the plant can be 
made, and as regards each heritable character 
the plant will be found to be either homozy- 
gous, heterozygous, or zeroz)-gous, and as such 
it will behave when bred from. Once the 
factorial analysis of a plant has been made, 
all future breeding from that plant is reduced 
practically to a certainty. 
Colour and AL^L^^ISM. 
Recent results show that m certain cases 
an apparently simple heritable character is 
due to the presence of more than one distinct 
factor. For instance, in Orchids we ha\'e 
already one case of that description, and no 
doubt there are many more }'et undiscovered. 
From evidence collected by the writer some 
