A MANUAL OF BOTANY 143 



ment furnishes the most luxuriant fern growth? Where, 

 for instance, are tree ferns abundant? I low do these ferns 

 differ from ferns of coUl regions? What nuist have been 

 the condition during the coal period for ferns to be so 

 abundant — in Illinois, for example — as to i)roduce, when 

 solidified into coal, from forty to one hundred feet thickness 

 of the same in twenty veins? How do hard coal, soft coal, 

 and lignite differ? Where are the coal fields of the world ? 

 There are nearly three thousand kinds of ferns. What uses 

 do you find for this large number of species ? 



Pteris aquilina (the common brake fern) is found in every 

 considerable land area on tlie earth. What would this show 

 concerning the ancient nature of this species, and of ferns 

 in general ? 



O' 



Class 2. Horsetails and Scouring Rushes 



General statement. Material for study may be easily o))- 

 tained in any region, but for practically all purposes, dried 

 and mounted specimens are as satisfactory as fresh. You 

 may have observed, in your spring walks, the fertile and 

 sterile stems of the common horsetail, in their place of 

 growth and their general appearance. Many students will 

 remember the scouring rushes as objects of use in childhood 

 play, under the names " snake grass " or " pipes." Examine 

 the horsetail, and observe the differences between t\\Q fertW> 

 aerial stem and sterile ; the former being known by its cone 

 of fruit at the summit. How do the two forms differ in 

 size, color, amount of branching, hardness of stem, and 

 size and appearance of the sheaths of scale leaves at the 

 nodes? Examine the branches of the sterile stem for 

 leaves of similar character. 



Leaves, in general, are the food-making organs of green 

 plants. What part must do tliis work in the horsetail? 

 Examine the stem (m, vlp). AN'hat features of the surface 



