110 PLANT PHYSIOLOGY 
uniting cells (gametes) may come from the same or 
from different plants, indeed they may be sister cells, 
i.e. formed by the division of one cell, but this is not 
common. They may be alike (isogamous) or unlike 
(heterogamous). 
158. As we proceed from the simple to more complex 
plants in the study of sexual reproduction we find entering 
in, the principle of “‘increased parental care.”’ In the 
lowest plants with sexual reproduction the gametes 
unite outside of the-parent plant, at a higher stage one 
gamete (the egg) is retained in the parent plant and is 
fertilized by the motile sperm. Still higher the egg is 
surrounded by special protective structures (cystocarp, 
archegone, etc.) and produces no longer a simple zygote 
but a spore fruit which may also be included in the pro- 
tective envelope. A still higher stage is where the 
spore fruit is so highly differentiated that it becomes a 
separate generation (sporophyte), capable of separate 
existence, similar to or differing in appearance from the 
parent generation (gametophyte). Highest of all we 
find the sporophyte becoming the prevalent generation, 
the gametophyte being retained within its protective 
tissues and only developing far enough to permit sexual 
reproduction to occur. 
159. Each gamete of the same species has the same 
number of chromosomes in its nucleus. The cell re- 
sulting from their union, the zygote, has double this 
number (diploid number). Where a zygote is formed 
which gives rise directly to a plant like the original one, 
the reduction in the number of the chromosomes from 
the diploid to the haploid number (see paragraphs 35 
and 160), occurs with the germination of the zygote. 
Where a spore fruit or sporophytic generation occurs its 
cells retain the diploid number and the reduction divi- 
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