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- 



