556 BOTANY OF THE LIVING PLANT 



has secured such advantages as follow from sex, through its retention 

 of the sexual generation within it. 



It thus appears that there are three forms in which somatic de- 

 velopment may be related to the nuclear cycle characteristic of 

 organisms possessing sexuality : (i) that in which a haploid soma 

 intervenes between the events of reduction and syngamy : this is 

 characteristic of haplobiontic Algae ; (ii) that in which there is 

 present not only the haploid soma as before, but also a diploid 

 soma which intervenes between the events of syngamy and reduction : 

 this is characteristic of the Archegoniatae, and the derivative Seed 

 Plants ; it is also seen in some Thallophytes, and is typical of the 

 diplobiontic Algae ; and (iii) that in which a diploid soma intervenes 

 between the events of syngamy and reduction, there being no haploid 

 soma : this is characteristic of Animals, and of certain Algae, e.g., 

 Fucus, and members of the Siphonales (Green Algae). The fact that 

 a haploid soma may exist without the diploid, while a diploid may 

 exist without the haploid, appears to strengthen the view that the 

 two phases seen in the Archegoniatae differ in the history of their origin : 

 and to support for them the theory of interpolation, of which that 

 historical difference is an essential feature. It is a recognised principle 

 in Morphology to fix the attention upon those features that are most 

 constant in their occurrence, and to accord to them the higher impor- 

 tance. Syngamy and its complementary Reduction thus take a prior 

 place, while somatic developments appear as inconstant incidents in a 

 more stable nuclear cycle. 



Summary. 



The sexual act probably originated polyphyletically among primitive Algae, 

 in some of which an early differentiation of the gametes can be traced by com- 

 parison with allied forms (Fig. 275). The two leading events of the sexual 

 cycle are Syngamy and Reduction. In haplobiontic plants such as the 

 Characeae they follow in direct succession. Here reduction takes place in the 

 first division of the zygote : in Chara the zygote develops directly into a new 

 haploid individual like the parent. There is thus no alternation in the full 

 Hofmeisterian sense. 



What gave rise to the Hofmeisterian Cycle appears to have been a post- 

 poning of the event of reduction and, by the interpolation of a diploid soma, 

 the fitting of a new phase into the cycle of life. This innovation once esta- 

 blished was seized upon in the Evolution of Plant Life on Land ; for it brought 

 important advantages, which are detailed on p. 547. The final result has been 

 the establishment of a Land Flora such as we see living on any land surface 

 today ; and it is dominated by the diploid soma. 



