—1 16- 



th e frond. Our physiological exceptions are Psomiocarpa 

 and Stenosemia, the vegetative and reproductive fronds 

 of which are distinct, and Lecanopteris, which may not 

 be entirely dependent upon photosynthesis for its or- 

 ganic food. For the sake of facile nutrition and to pre- 

 serve the normal exercise of its functions by the nether 

 epidermis, the sporangia of practically all ferns whose 

 vegetative and reproductive fronds are alike are collected 

 into sori. Most ferns protect their sori by means of in- 

 dusia. At San Ramon, sixty per cent, of all the Polypo- 

 diacese have indusia, the remaining forty per cent, in- 

 cluding thirteen members of the old genus Acrostichum 

 and a number formerly put in Gymno gramme , besides all 

 those with well-defined nude sori. Any full discussion 

 of the forms and origin of indusia would be superfluous 

 here in view of the attention they have received as most 

 important structures in taxonomy, but it is pertinent to 

 the subject of this work to point out that their structure 

 fits the local demands upon it. Thus, it is leathery in 

 the two strongly marked xerophytic genera, Davallia and 

 Humata, but not in their mesophytic relatives. In As- 

 plenium it is the xerophytic section with fronds which 

 has by far the firmest indusia. 



A heavy coating of hairs protects the sori as well as 

 the stomata against undue loss of water in Niphobolus 

 lingua and various congeners. While the function of 

 paraphyses in general is to protect against water rather 

 than dessication, there are some ferns, the paraphases 

 of which cover the sporangia so thoroughly that they 

 must serve in their time in both ways. The protection 

 of the sorus by the folding backward of the margin of 

 the frond is familiar to all in the indusia of most Pteri- 

 dese. The same effect is reached very thoroughly by 

 two of our species of Poly podium — P. ciicullatum and P. 

 gracillimum — which have one-half of each pinna wholly 

 or partly folded backward against the other half, cov- 

 ering the single sorus. In Acrosorus the folding is com- 



