288 Science of Plant Life 



from the prostrate stems. The stems bear large numbers of 

 scale leaves, and the spores are produced in terminal cones. 

 The spores are sold in drug stores under the name " ly co- 

 podium powder." They are used as a drying powder and 

 in the manufacture of fireworks. 



The club mosses were of large size and very abundant 

 during the time when the carboniferous rocks were deposited, 

 and with the ferns and equisetums they formed the chief part 

 of the forests of that geological period. The coal which we 

 now bum is the last remnant of the carbon compounds formed 

 by plants from the carbon dioxid then present in the atmos- 

 phere. Certain of the club mosses, like some of the ferns, 

 show stages in the transition from reproduction by spores 

 to reproduction by seeds. The club mosses and equisetums 

 have highly branched stems and many small, scalelike leaves. 

 In the ferns there is httle branching of the stem and the leaves 

 are large and often much divided. 



The two generations of the ferns. One of the most inter- 

 esting features of the hfe history of the fernlike plants is the 

 fact that there are two very different plants produced during 

 one Hfe cycle. Asexual spores are produced on the under 

 surfaces of fern leaves, and when these are planted on soil 

 and allowed to germinate, we fitnd that in about 6 weeks the 

 soil is covered with small, heart-shaped thalli which look very 

 much like liverworts. These thalli represent one generation 

 of the fern plant. Each thallus consists of an irregular plate 

 of cells, a single cell layer in thickness except toward the 

 middle, where it may consist of several layers (Fig. 172). 

 The prothallus, as the plant is called in this stage, is anchored 

 to the soil by one-celled rhizoids. It manufactures its own 

 food and has an existence similar to that of a liverwort. 



