ii6 THE SCIENCE OF LIFE. 



minute spores may be observed by sowing them on a 

 saucerful of fine mold, covering with a bell-glass or tum- 

 bler, and keeping it moist, warm, and shaded. A green 

 film will spread over the soil, which can be taken up, 

 from time to time, on the point of a knife for microscopic 

 examination. The little spore swells and bursts, and 

 throws out a rootlet which gets its nourishment from the 

 soil. Then a number of delicate cells are formed from 

 the mother-cell in the spore, making a little green scale, 

 (the prothalliu m,) which throws out rootlets on its under 

 side. This prothallium produces two kinds of cells, one 

 set called antheridia, which contain spiral filaments which 

 escape to enter the others, called archegonia, or germ- 

 cells, from which the future fern is reproduced. (Fig. 46.) 



The fossil remains of Ferns in the various strata of 

 the earth's crust are very numerous, especially in the 

 Coal measures. These deposits exhibit the remains of 

 many species now extinct. Immense tree-ferns and gi- 

 gantic Lycopodiaceae (Club-mosses) flourished in an at- 

 mosphere charged with moisture and carbonic acid gas, 

 which, by plant assimilation and liberation of oxygen, is 

 thought to have been purified and prepared for the use 

 of successive tribes of animals and of man. 



5. MOSSES are minute and lowly plants, but they are 

 by no means insignificant. There are about ten thou- 

 sand species, some of which are not over a hundredth 

 part of an inch in height, while others are several inches 

 high. Mosses have a distinct axis of growth, and the 

 delicate leaves are arranged with great regularity. The 

 stem shows some indication of the separation of a cor- 

 tical, or bark-like portion, from the medullary, or pith- 



