132 TYPE STUDIES 



sexual generation (gametophyte) ? Are there spores among 

 the algae comparable to the moss spores ? Is it a new type 

 of spore in plant evolution ? What is the physiological relar 

 tion of the sporophyte to the gametophyte ? What impor- 

 tant advance in the structure of the moss sporophyte over 

 that of Marchantia is indicated by the presence of stomata ? 

 In what other respects does the moss sporophyte show ad- 

 vances over that of Marchantia ? Describe the life history, 

 distinguishing between the sexual phase, gametophyte, and 

 the asexual phase, sporophyte. Draw and arrange a series 

 of diagrams illustrating the chief stages throughout the life 

 history, using two colored pencils to designate the gameto- 

 phytic and sporophytic generations respectively (App. 18). 

 Construct a life-history formula that will express this suc- 

 cession (App. 18). 



THE FEENS, OE FILICINE^ 



129. Field work on the ferns, horsetails, club mosses, and quillworts. Bepre- 



sentatives of these groups have well-defined habits, some living under very- 

 special life conditions and others forming striking and characteristic associ- 

 ations (see Principles, p. 475), such as the growths of bracken fern (Pteris aqui- 

 lina), many horsetails, and some club mosses and quillworts. These may be 

 studied in the field and the habits of growth and life conditions noted. The 

 adaptations of the pteridophytes are extremely various. There are hydro- 

 phytes, as some species of Marsilia and Isoetes, and floating aquatics, such as 

 Azolla and Salvinia. Numerous mesophytes occur especially among the 

 ferns, as illustrated by species of Aspidium, Asplenium, and Cystopteris. 

 Xerophytes are represented in the ferns by such forms as Gymnogramme in 

 the far Southwest and Polypodium incanum in the South. Equisetum and 

 certain species of Selaginella are excellent types of xerophytes, and some 

 species of the latter may remain perfectly dry for long periods and will re- 

 vive again when wet. Field work on the pteridophytes should be planned 

 with reference to the ecological relations of the forms and may be accom- 

 panied with great advantage by morphological and histological studies that 

 will show the character of the adaptations in the plant's structure. In tak- 

 ing up the pteridophytes the student passes to a group whose life relations 

 approach those of the seed plants, and they may be studied in the field largely 

 after the same methods as the latter group (see Sec. 157). The displays 



