ALTERNATION OF GENERATIONS 545 



of gametes. The cycle thus constructed may be represented as in 

 the diagram, Fig. 427. Since the two phases follow one another in 

 regular succession, this phenomenon is that which is now understood 

 as the normal Alternation of Generations in Plants. It is also referred 

 to as the Hofmeisterian Cycle. 



The deviations from the normal cycle known as apogamy and apospory 

 have been illustrated by examples from Ferns, pp. 506-509. They connote, 

 respectively, a direct vegetative transition from the gametophyte to the 

 sporophyte without the act of syngamy, and a converse transition from the 

 sporophyte to the gametophyte without the intervention of spores. Though 

 recorded instances of such deviations from the normal are numerous, they are 

 not standardised as the normal cycle is in archegoniate plants. This suggests 

 that they are ex post facto events, illustrating the potentialities of plants 

 at the present day rather than evolutionary features forming part of the 

 history of the past. 



It seems probable in many, though not perhaps in all phyla, that 

 nuclear fusion and reduction remained constant features in each com- 

 pleted life-cycle throughout Descent. In that case two opportunities 

 for somatic amplification were possible in Evolution from simpler 

 forms. The one between syngamy and reduction would give rise to the 

 body of tissue called the sporophyte : the other between reduction and 

 syngamy would give rise to the gametophyte. If the fusion and reduc- 

 tion retained their identity throughout Descent, these two somata can 

 never have been homologous : that is, homogenetic by descent. They 

 must have been " antithetic " throughout, however nearly they may 

 resemble one another in their characters. If they both develop in the 

 same medium they, being in fact merely phases of the same organism, 

 might be expected to resemble one another very closely. 



That is found to be actually the case in certain Algae and Fungi. For in- 

 stance, in Dictyota among the Brown Algae, and Polysiphonia among the Red, 

 the two generations appear identical (pp. 383-390) ; they seem only to differ in 

 their chromosome-number, and in the propagative organs which they bear. 

 There may be a difference of potentiality between the haploid and diploid 

 phases ; but it need not be realised where the circumstances are uniform, as 

 when both grow in water or in moist conditions. In that case both may appear 

 alike. But even in an aquatic environment the two generations may show 

 marked differences as in Laminaria. 



The Hofmeisterian Cycle. 

 The inconclusiveness which marked the early discussions on alternation 

 arose partly from an assumption that what is seen normally in Ferns or Ifom 

 and described as the Hofmeisterian Cycle, is a standardised life-pattern for 

 plants at large. There was also insufficient knowledge of the facts of nuclear 

 structure and behaviour, particularly in the Thallophytes. But the most 

 notable defect was the failure to take into account the diverse conditions to 



