ALTERNATION OF GENERATION 547 



sexuality for propagation, and some alternative method oi ina aid 



be an advantage. This was provided by interpolation 0/ a new ft phase, 



the act of reduction being delayed by progressive Sterilisation and developn 



of the new sporophytc. The natural result would be a multiplicity of 

 mother-cells, and a higher spore-output: moreover, sue h accord with 



the known facts of life among the simplest archegoniate plants. spore 



dispersed under dry conditions starts a new individual, yet all those produ< 

 by one sporophyte would be consequent on a single act oi fertilisation. The 

 organism thus leads an amphibious existence, tied down by occasional fer- 

 tilisation through water, but with the resulting spores numerous, and d 

 seminated in dry air. 



The idea of this interpolation of the sporophyte as a factor in the evohr 

 of archegoniate plants is not new. It was first brought into prominence 

 by Naegeli as illustrating a principle of evolution in plants in his Abstam- 

 tnungslehre of 18S5, though the fact of sterilisation of fertile cells to form 

 additions to the sterile tissues in a plant possessing sexuality was well known 

 at an earlier date (see Chapter XXXIV. : also Bower, Primitive Land Plants, 

 Chapter XXIV., pp. 489-491). Such sterilisation was formulated by Naegeli 

 thus : he states that " the phenomenon of reproduction of one stage becomes 

 at a higher stage that of vegetation. The cells which in a simple plant are set 

 free as germs and constitute the initials of new individuals, become in the next 

 higher plant part of the individual organism, and lengthen the ontogeny to a 

 corresponding extent" {Abstammiingslehre, 1885, p. 352). 



Such an interpolation of a sporophyte, hypothetically sketched here, is 

 historically probable. Not only would an increased fertility be secured 

 under variable circumstances but also, as Svedelius has pointed out, the 

 multiplication of delayed acts of reduction will increase the number of nuclear 

 combinations, and the plant thereby multiplies its chances of forming new 

 heritable characters. Given conservatism in the mode of fertilisation, which 

 is assumed to be inherited from an algal ancestry, an interpolated alternation 

 would be biologically stabilised as an outcome of migration to land. There are 

 no close links connecting any living green Algae with any archegoniate 

 plants. The often-quoted genera, Ulothrix and Coleochaete, as also the isolate >1 

 class of the Characeae, all stand on the haplobiontic plane, reduction being 

 involved in the first division of the zygote. None of them has hit off the in- 

 novation of postponing reduction by the interpolation of a diploid soma fitted 

 for subaerial life. The focal point of an interpolation theory is that certain 

 littoral organisms did this. Once the step is taken the wide morphological gap 

 between aquatic and subaerial vegetation explains itself : for the advancing 

 archegoniate type would by originating a diploid soma have achieved three 

 biological ends of supreme importance to any land-living organism : (1) a 

 multiplication of possible combinations of heritable characters, giving the n 

 material for variation .; (ii) an opportunity for wide dissemination by air-borne 

 spores ; and (iii) relief from dependence on repeated syngamy for numerical 

 increase. The last was probably the most important for land-living plants. 

 The superiority thus gained by the early plant-amphibians will have favoured 

 the advance of the interpolated sporophyte, and the haplobiontic algal ancestors 

 would be left hopelessly behind. Hence the wide gap between any green Alga 

 and any living archegoniate plant. 



