Feb., 1922 sexual evolution in the plant kingdom 111 



eration might perhaps define the condition somewhat better, 

 but objections might also be raised to this term as apphed 

 to heterosporous sporophytes. If we designate individuals by 

 the kind of reproductive cells they produce, then there is no 

 confusion in designating the sporophyte as "nonsexual." It is 

 probably best at present to hold that the low^est forms are 

 entirely nonsexual and not merely neutral, because their proto- 

 plasts apparently do not have the proper complexity of organ- 

 ization which would permit of or lead to sexual states. 



The first differentiation then, as shown above, is in the 

 gametes themselves, and apparently the most fundamental 

 manifestation of sexuality is one of reaction or attraction of 

 two cells (gametes) toward each other. If the sexual state 

 arises in a cell, destined to be a gamete, after its morphological 

 expression is practically completed then there can be no struc- 

 tural dimorphism, but the earlier the maleness or femaleness is 

 established in the cell the more extreme the dimorphism must 

 become. Since there can be only a short transition between the 

 two extremes of isogamy and heterogamy the normal hetero- 

 gamous state is soon attained, often with little or no advance 

 in the evolution of vegetative parts, as stated before. The 

 typical egg as has long been known, is comparatively large with 

 a large amount of cytoplasm and food content and is nonmotile, 

 lacking cillia and flagella, while the sperm is small and motile, 

 being provided with cilia or flagella. These distinctions are true 

 for the great majority of types of plants and animals, from the 

 lowest heterogamous species to the highest. The greatest 

 deviation is in respect to the motile organs of the sperm, which 

 are absent in the rhodophyta, strobilophyta, and anthophyta. 

 In other respects, however, the typical dimorphism is not 

 materially altered. Secondary sexual characters begin to appear 

 when a given sexual state is established in the neutral veg- 

 etative cells whose descendants are not all transformed into 

 gametes or when a given sexual state is established in cells 

 which are destined to produce vegetative structures only or at 

 least before the final gamete-producing tissue is developed. 

 Finally the dimorphism may involve the entire individual. In 

 this case the sexual state must be present in the spore from 

 which the individual develops or soon after its germination. 

 The conditions which determine whether a cell or tissue shall 

 pass into the male or female state are probably diverse, but 



