286 The Living Plant 



each chromosome in cell division, carries an implication of great 

 importance to an understanding of the nature of sexual reproduc- 

 tion, namely, it implies that both parents contribute exactly alike 

 to the characteristics of the offspring, the selection between the 

 double set of paternal and maternal characters being made in the 

 course of development of the offspring itself. This view is dia- 



FIG. 100. A diagrammatic representation of fertilization, showing the passage of the male 

 nucleus from the pollen tube into the egg-cell, and its fusion with the nucleus thereof. 

 The black rods in the nuclei are chromosomes, described on page 284. 



metrically opposed to the older idea, once advocated by some 

 biologists, that each parent contributes something the other does 

 not; and it is obviously quite different from the various popular 

 notions, which, naturally, are largely erroneous. 



But now there arises this fundamental question. If the two 

 sexes contribute substantially alike to the offspring, why are they 

 not substantially alike in structure? What is the meaning of the 

 differences between the sexes? Or, to go a stage deeper, why does 

 sex exist at all? Happily these questions can be answered with 

 reasonable certainty through evidence supplied by a study of 

 existing transitions from the simplest plants, where sex has not 

 yet developed, to the highest plants and animals, where it is fully 

 differentiated. Thus, there exist some simply-organized seaweeds 

 which throw out into the water a great many reproductive cells, 

 all exactly alike and provided with suitable structures for swim- 

 ming (figure 102). These move towards one another and come 

 together in couples, which then fuse completely, uniting their 

 nuclei; and thus is formed a " fertilized" cell which gives origin 

 to a new plant precisely as does a fertilized egg-cell. Obviously 

 fertilization in this case occurs between sexual cells precisely alike; 



