THE ORIGIN OF SEX IN PLANTS 21 



being motile, collect around them in large numbers, coming 

 into close contact with their surface (Fig. 10 (5)). One 

 of them penetrates into the egg, and its nucleus has been 

 followed in its course inwards to the central nucleus of 

 the egg (Fig. 10 (6) ). Finally the two nuclei fuse together. 

 These details have been successfully observed by Professor 

 Farmer, who notes how immediately after the entrance 

 of the fertilizing spermatozoid into the ovum all the rest 

 leave it, as though repelled by some common impulse. 

 The fertilized egg, or zygote, may then grow directly into 

 a new Fucus plant. 



This series of Brown Seaweeds probably indicates the 

 general course which the differentiation of sex has taken 

 in primitive Plants. The strength of the argument that 

 it does so lies not only in the gradual steps which they 

 show, but also in the fact that these observations do not 

 stand alone. Other large groups illustrate the .same 

 thing. For instance, those closely related Green Algae 

 which are called the Siphonales and Siphonocladiales, 

 afford a series of steps which are quite comparable with 

 those seen in the Brown Seaweeds. Starting with the 

 conjugation of equal gametes, as it is seen in Acetabularia 

 (Fig. n, i), we arrive by steps of increasing inequality in 

 size and behaviour at the complete distinction of sex seen 

 in Vaucheria (Fig. n, v). Here, as in Fucus, there is a very 

 large immobile egg, which is fertilized by a very small 

 motile spermatozoid. Other groups show also a like 

 progression from isogametes to spermatozoid and ovum, 

 distinguished by size and behaviour. Such progressions 

 may be matched in Animals as well. 



The question naturally arises why such progressions 

 should appear in several distinct evolutionary lines. That 

 the differentiation of sex has occurred more than once 



