THE FERX BULLETIN 



17 



that the characters of the indusium be given second 

 place in distinguishing species. L. M. Underwood 

 finally came to hold the opinion that the veins and the 

 ribro-vascular structure of the plants in general should 

 have chief consideration. The latest student to at- 

 tack the problem is C. Christiansen, author of "Index 

 Filicum," who proposes to separate the species into 

 groups based upon the character of the hairs and scales 

 they bear. Underwood's greatest objection to the in- 

 dusium as a distinguishing mark was that it is a mere 

 scale growing out of an epidermal cell, but the posi- 

 tion of Christiansen is still worse for he relies upon 

 mere plant hairs which are well known to vary in 

 color, size and abundance in the same individual as 

 any but a systematic botanist is well aware. If the 

 problem was only one that concerns the few species in 

 a given region, their separation presents few difficul- 

 ties, but the more species we have the greater the per- 

 plexities encountered. In the tropics one meets with 

 plants that would at once be referred to Nephr odium 

 and not far away, others extremely like them but lack- 

 ing an indusium while still others may have an in- 

 dusium for a few brief days when the sporangia are 

 developing. All this shows the unreliable nature of 

 the inciusium as a distinguishing character and throws 

 us back upon the suggestion of both Gilbert and Un- 

 derwood that structure and habits of growth are the 

 best guides. Our greatest trouble is that nature did 

 not attempt to make genera and therefore any group 

 that man can devise must have points at which it 

 intergrades with other groups. Many of the differ- 

 ences between fern students are due to the inability to 

 agree upon where the dividing lines should be drawn 

 and in no group is this better illustrated than in the 

 wood ferns. 



