122 BOTANY. 



the fern bundle. There is no individual bundle sheath, but the whole 

 circle of bundles has a common outer sheath. 



The epidermis is composed of elongated cells whose walls present a 

 peculiar beaded appearance, due to the deposition of flint within them. 

 The breathing pores are arranged in vertical lines, and resemble in general 

 appearance those of the ferns, though differing in some minor details. 

 Like the other epidermal cells the guard cells have heavy deposits of flint, 

 which here are in the form of thick transverse bars. 



The spore cases have thin walls whose cells, shortly before maturity, 

 develop thickenings upon their walls, which have to do with the opening 

 of the spore case. The spores (Z7, 7) are round cells containing much 

 chlorophyll and provided with four peculiar appendages called elaters. 

 The elaters are extremely sensitive to changes in moisture, coiling up 

 tightly when moistened (7), but quickly springing out again when dry (77). 

 By dusting a few dry spores upon a slide, and putting it under the micro- 

 scope without any water, the movement may be easily examined. Lightly 

 breathing upon them will cause the elaters to contract, but in a moment, 

 as soon as the moisture of the breath has evaporated, they will uncoil 

 with a quick jerk, causing the spores to move about considerably. 



The fresh spores begin to germinate within about twenty-four hours, 

 and the early stages, which closely resemble those of the ferns, may be 

 easily followed by sowing the spores in water. With care it is possible 

 to get the mature prothallia, which should be treated as described for the 

 fern prothallia. Under favorable conditions, the first antheridia are ripe 

 in about five weeks ; the archegonia, which are borne on separate plants, 

 a few weeks later. The antheridia (Fig. 72, J, an.) are larger than those 

 of the ferns, and the spermatozoids (K) are thicker and with fewer coils, 

 but otherwise much like fern spermatozoids. 



The archegonia have a shorter neck than those of the ferns, and the 

 neck is straight. 



Both male and female prothallia are much branched and very irregular 

 in shape. 



There are a number of common species of Equisetum. Some 

 of them, like the common scouring rush (E. hiemale), are un- 

 branched, and the spores borne at the top of ordinary green 

 branches ; others have all the stems branching like the sterile 

 stems of the field horse-tail, but produce a spore-bearing cone 

 at the top of some of them. 



