MARSILIA 139 



designate the gametophytic and sporophytic generations 

 respectively (App. 18). Construct a life-history formula that 

 will express this succession (App. 18). 



131. Fronds, vegetative leaves, and spore leaves, or sporophylls. Study the 

 fronds of such fenis as Onoclea sensibiUs, or O. struthiojjteris, or Osmunda 

 ciunamoinea. Osmunda regalts and 0. Claytoniaua illustrate the points out- 

 lined below to a partial extent. Herbarium material, — that is the fronds 

 mounted on sheets, — may be used. Observe that there are two forms 

 of fronds : 



A. Vegetative fronds, which may be called vegetative leaves, or sim- 

 ply leaves. 



B, Spore-bearing fronds, which are called spore leaves or sporojjhylls. 

 The sporophylls are devoted almost entirely to the work of spore 

 production. They are not expanded; there is relatively little green 

 tissue, and they are not well suited to the work of photosynthesis. 

 Thus the fronds of these ferns have become differentiated into two 

 sets, one devoted to the purposes of spore production and the other to 

 vegetative activities. 



Examine a series of leaves illustrating the general conditions stated 

 above. Compare them and draw. Search for leaves with mixed char- 

 acters, that is, in parts devoted to spore production, and in other parts 

 strictly vegetative. These are not uncommon, and their study makes 

 clearer the relationship between the vegetative leaves and the spore leaves 

 or sporophylls. Ophioglossum, Botrychium,Aneimia, and Lygodium illus- 

 trate excellently the mixed characters noted above, but material is less 

 likely to be available. Some of the common ferns — as the Christmas 

 fern {Polystichum acrostichoides) — also show the same tendencies. 



132. Marsilia, a water fern.* Study the life habits of the plant 

 as it grows in the water or in wet situations. 



A. General morphology. Note : 



1. The creeping stem, or rootstock (rhizome). 



2. The four-parted leaves. Contrast their veining with that 

 of the clover leaf. 



3. The fibrous roots. 



* To THE Instructor : If only one heterosporous pteridophyte can be studied 

 in the laboratory, it is perhaps better that the form be Selaginclla. But the ease 

 with which the spores of Marsilia (especially M. vestita) may be pjerniinated and 

 the gametophytes obtained, together with the excellent study of .sperms and the 

 young sporophyte, which is offered, make it an especially attractive type lor 

 study, and it should be included in a general course whenever possible. 



