410 VEGETATION OF THE CARBONIFEROUS PERIOD 



caudex previous to fossilization, would effectually prevent the two parts 

 being recognised as portions of one plant. 



The tendency in this case is towards the multiplication of species, 

 while in the former we should unite what are nearly totally distinct. 



Venation. In the absence of those characters afforded by fructification 

 which are acknowledged to be of the very highest value in this order, 

 if indeed they are not the only ones by a study of which the primary 

 groups of ferns are to be united, the venation, or arrangement of the 

 nerves, is adopted. This character is considered by many of secondary 

 importance, and one by which the primary groups may be divided 

 into minor groups without violence to nature, especially according to 

 whether the veins after branching join again, or continue free to the 

 margin of the frond. Now, there are doubtless many sub-orders both 

 of fossils and recent ferns distinguished by the organs of fructifica- 

 tion, each of which again is sub-divided by characters in the venation ; 

 but instead of adopting the primary characters (of fructification) in 

 the formation of these sub-orders, the secondary ones (of venation) 

 are employed : it follows, then, that every such group (and they are 

 genera) of fossil ferns may contain species generically widely apart. 

 The genera must, of course, be wholly artificial, and include plants 

 belonging to separate sub-orders, as would be proved did their fructifi- 

 cation occur. 



Even venation is a character to be used with caution ; for though the 

 presence or absence of an involucre to the sorus is constant in one 

 species, and so are the form and position of the sori, the venation is found 

 occasionally to vary materially in different parts of the same plant, or 

 even the same frond. Hence, while some sub-orders of ferns may be 

 trenchantly divided according to the branching of the nerves, others 

 cannot, because the individual species at times assume both forms in 

 one frond. 



This is remarkably the case with the Diplazium Malabaricum* of 

 whose fronds two states are figured (Fig. 7). In the upper and older 

 portions it will be seen that the veins, after branching, meet ; while in 

 the lower the branches run free to the margin of the frond. Fossil frag- 

 ments of this fern would be included under two genera, those as resem- 

 bling Fig. A to Callipteris, and those veined in B to Digrammaria. It 

 would be out of place to exhibit through what transition stages the pas- 

 sage between A and B is effected : any one frond, however, of this par- 



* This plant has caused no little perplexity to the students of existing ferns. So pro- 

 tean are its fronds, that different states of this one fern hare been made into two separate 

 genera, Anisogramma, Pres., and Digrammaria, Presl. Mr. John Smith, of Kew, to 

 whom I owe many illustrative specimens of these points, has separated this plant from 

 Diplazium, and given it the name of Callipteris. 



