HEEEDITY 



surface of its fronds. Each of those spores 

 is a reproductive cell which, like the mature 

 eggs and sperm of animals, is in a reduced 

 nuclear condition (N). These spores germi- 

 nate, however, without uniting in pairs and 

 form a plant different from the parent, just 

 as the mature egg of a bee, if unfertilized, 

 develops into an individual different from the 

 parent, in that case a male. The plant which 

 develops from the spore of a fern is small and 

 inconspicuous and is known as a prothallus. 

 See Fig. 7. It produces sexual cells (eggs and 

 sperm) which, uniting in pairs, form fern-plants, 

 2 N individuals. Thus there is a constant alter- 

 nation of generations, fern-plants (2 N), which 

 produce prothalli (N), and then these produce 

 again fern-plants (2 N). 



The fact is worthy of note that in an animal 

 or plant which is in the single or N condition, 

 there occurs no chromatin reduction at the 

 formation of reproductive cells. Its cells are 

 already in the single condition, and they 

 probably cannot be further reduced without 

 destroying the organism. The 2 N fern-plant 

 forms reproductive cells, its spores, which are 



22 



