310 NATURAL SCIENCE [November 



plant, or, better, the ' plant ' of the Scale-moss or Liverwort, to 

 which it bears a close resemblance. On this are borne sexual 

 organs which produce sexual cells. The coupled-cell, as in the Moss, 

 is at first parasitic on the scale, and develops into a Fern ' plant,' 

 such as we know it with stem, roots, and leaves, and finally spores. 

 The essential difference here is that in the Mosses the spore-forming 

 plant is entirely parasitic and of limited growth, while in the Fern it 

 becomes independent, and is of unlimited growth, being provided 

 with organs of support and conduction as well as of nutrition. We 

 may well say that the sterilisation (to use Bower's term) of part of 

 the colony has led to so extended a power of colonial growth and 

 branching, that the power of forming reproductive cells is in the end 

 enormously increased. The propagative capacities of Ferns by buds 

 from embryonic tissue are very great ; those of fragments of the 

 spore-bearing plant are slight ; but the sex-bearing scale may be 

 artificially propagated by being cut into small pieces, although its life 

 is usually limited by the formation of the parasitic Fern-plant from 

 the coupled-cell. 



Ferns then show the same alternation between spore-bearing and 

 sex-bearing generations as Mosses, but the order of relative con- 

 spicuousness and abundance of colonial growth is inverted. We 

 have seen that in Mosses a vegetative transition by cell growth 

 might take place from the spore-bearing generation to the other. 

 In Ferns similar transitions are possible both ways, so as to cut out 

 the stage of brood cell formation, which we regard as the critical 

 reproductive stage. 1 Thus in many Film-ferns, instead of producing 

 spores, the leaves grow out into scale-plates bearing sexual organs, 

 while in the common Cretan fern, the scale produced from the 

 spore grows out directly into the spore-bearing leafy Fern-plant 

 instead of giving rise to sexual cells. In flowering plants the 

 relations of the sex-bearing plant are much obscured, and it 

 would lead us too far to explain them here. Suffice it to say 

 that the 'plant' as we know it corresponds to the Fern-plant 

 or moss-capsule : it is the Sporophyte, not the Gametophyte. The 

 parasitism of the embryo formed from the coupled-cell is usually 

 intense and prolonged. 



A very remarkable character of Dicotyledons or Exogens is the 

 continuation downwards from the growing point of a zone of em- 

 bryonic tissue, the 'cambium,' which habitually by its growth and 

 multiplication forms zones of wood on the inner side, and inner-bark 

 (or bast) on the outer. This layer has, in cuttings, an especial tendency 

 to form buds. But all the living cells retain a power of forming a 

 similar tissue at or near an exposed surface ; for instance, such a 

 layer is formed a little within the surface of trees to produce the 



1 These transitions have been aptly termed "short circuitings " by Sir Edward Fry. 



