Physical Basis of Heredity and Evolution 



29 



methods of reproduction recognized among plants. The 

 most primitive method is vegetative multiplication, ordinary 

 vegetative cells producing new plants. In the transmission 

 of hereditary qualities this involves 

 simply a series of such cell divisions 

 as has been described above. 



Later in the evolution of plants, 

 the power of reproduction was dis- 

 played chiefly by spores, which at 

 first were only certain protoplasts 

 that escaped from the incasing wall. 

 In most cases, before escape the 

 protoplast divides, so that there 

 issue from the old cell two or more 

 naked protoplasts or spores. Since 

 the early spores belonged to water 

 plants, they had swimming append- 

 ages (cilia), and were called swim- 

 ming spores or zoospores (Fig. 3, a 

 and b). Any one of these swim- 

 ming spores, under appropriate 

 conditions, can produce a new in- 

 dividual. This method of multi- 

 plying individuals remains the most 

 effective method among plants. 



Vegetable multiphcation and 

 reproduction by spores are both sexless methods, and it is 

 quite e\'ident that the introduction of the sexual method is 

 more significant than merely a third method of reproduction. 



It is demonstrable among plants that the sexual cells 

 (gametes) were derived from the swimming spores (zoospores). 

 In certain plants, if a protoplast di^'ides and gives rise to 



Fig. 3. — A portion of a fila- 

 ment of a green alga (Ulotlirix) : 

 a, zoospores in mother cell; b, 

 an escaped zoospore; c, ga- 

 metes, most of which have 

 escaped from the mother cell; 

 d, gametes pairing and fusing; 

 f, zygotes. 



