6 FERN GROWING 



examine a number of plants of other countries, we obtain a 

 gradation that reduces Athyrium to a subsection of AspUnium. 



The English Polystichums are also placed as a subsection 

 of Aspidium, retaining, with Moore, the three species, Lonchitis 

 aculeatum, and angulare, in preference to uniting the two latter. 



When two species have been crossed, the progeny has 

 invariably been more or less sterile : there is, however, no such 

 thing as complete sterility ; when the offspring is confined 

 to one or two, sterility must be more marked than when 

 dealing with species where the offspring would be in thousands. 

 We shall return on a future page to this subject ; it is only 

 essential at the present time to state that in crossing Aspi- 

 dium angulare with Aspidium aculeatum there is found to be 

 so near an approach to sterility as to warrant us in saying 

 that A. aculeatum and A. angulare are distinct species. The 

 difference between species and varieties is evidenced by the 

 difficulty of crossing between individuals the first together ; 

 whereas with the latter there is no difficulty. 



As regards the British Lastreas being placed as a 

 subsection of Nephrodium, it seems that the peculiarities are 

 too slight to make Lastrea and Nephrodium into two distinct 

 families ; adopting Lastrea as a subsection is as far as we 

 can go. 



With, respect to N. Filix-mas, however, there can be no 

 doubt that Wollaston was correct in dividing this Fern into 

 three species ; it is as difficult to cross any of these three 

 together as it is with that of any other two species. 



Wollaston gave the names of Filix-mas, pseudo-mas, and 

 propinqua, the first deciduous, the second sub-evergreen, and 

 the third a mountain species. Wollaston's names, however, 

 have not been retained, as his pseudo-mas is Nephrodium 

 paleaceum, i.e., the Aspidium paleaceum of Don, whilst his 

 propinqua is the Aspidium abbreviatum of Poiret ; the older 

 names have therefore been adopted. Still our thanks are 



