and Laboratory Methods. 2571 



Laboratory Outlines for the Elementary Study of Plant 



Structures and Functions from the Standpoint 



of Evolution. 



XLIX. Mosses, Genera/ Study. Class, Musci. Order, Bryales. 



(a) 77ie young gametophyte. 



When the nonsexual spore of a moss germinates it does not give rise directly 

 to the scaly gametophore, but develops a green filamentous pro-embryo known as 

 the protonema. The protonema can always be found in connection with the 

 very young moss plants which are usually present in greenhouses. The proto- 

 nema may also be found by examining some of the black earth from a place 

 where mosses are growing. The ripe spores of any common species of moss 

 may be sown on moist soil in a box. In a few days, if the box has been cov- 

 ered with a pane of glass, an abundance of green filaments will begin to appear. 



1. Place a little earth with young moss plants into a watch glass and care- 

 fully wash off the soil by means of the medicine dropper and needle. Mount 

 the plantlets and any minute masses of filaments present. Examine under low 

 power. Find a good protonema and draw. Notice the branching, the shape of 

 the cells, and the chloroplasts. The similar brown filaments present are rhizoids. 



2. Draw a single cell, showing the wall, the cytoplasm, and the chloroplasts. 

 Notice the oblique walls which may be seen in the older filaments. Where and 

 how do the branches originate ? 



3. Find a protonema which has developed one or more solid green buds 

 from which the mature sexual moss plants (gametophores) will develop. Draw. 



4. With what kind of plants previously studied does the protonema com- 

 pare ? What then could you call the protonemal stage ? How can this be used 

 to explain the evolution of a moss as to habitat, form, and structure ? Explain 

 its evolution on this basis; remembering that the protonema is (1) a single cell, 

 (2) a simple filament, (3) a branched filament ; and that (4) it finally develops 

 solid buds. These four stages represent the four successive steps in the evolu- 

 tion of the plant body in going from the lowest unicellular forms to the liver- 

 worts. Ontogeny is supposed to partly explain phylogeny. Learn the following 

 law : The history of the development of the individual is an abbreviated history 

 of the development of the race to which it belongs. 



(b) The young vioss plant {gametophore^. 



Physcomitrhwi turbmatum (nearly always abundant in greenhouses, by road- 

 sides, and in old fields) or a species of Mnium will be suitable. 



L Mount in water and sketch the entire frond under low power, showing 

 the stem, scales, and rhizoids. 



2. Draw a single scale, carefully showing the costa and the margin. How 

 does it differ from the scale of Porella ? Under high power draw a cell showing 

 the large chloroplasts and thick wall. As in the liverworts these scales are not 

 homologous with true leaves. 



