TYPES OE CRYPTOGAMS; PTERIDOPHYTES 287 



351. The Frond. Fern leaves are technically known as fronds. 

 Observe how these arise directly from the rootstock. 



Make a somewhat reduced drawing of the entire frond, which 

 consists of a slender axis, the rhachis, along which are distributed 

 many leaflets or pinnce, each composed of many pinnules. Draw the 

 under side of one of the pinnae, from near the middle of the frond, 

 enlarged to two or three times its natural size, as seen through the 

 magnifying glass. Note just how each pinnule is attached to its 

 secondary rhachis. 



Examine the under side of one of the pinnules (viewed as an 

 opaque object without cover-glass) with the lowest power of the 

 microscope, and note : 



(a) The "fruit-dots" or sori (Fig. 210, B) (already seen with the 

 magnifying glass, but now much more clearly shown). 



(5) The membranous covering or indusium of each sorus (Fig. 

 210, C). Observe how this is attached to the veins of the pinnule. 

 In such ferns as the common brake (Pteris) and the maidenhair 

 (Adiantum) there is no separate indusium, but the sporangia are 

 covered by the incurved edges of the fronds. 



(c) The coiled spore-cases or sporangia, lying partly covered by 

 the indusium. How do these sporangia discharge their spores? 



Make a drawing, or several drawings, to bring out all these points. 



Examine some of the sporangia, dry, with a power of about fifty 

 or seventy-five diameters, and sketch. Scrape off a few sporangia, 

 thus disengaging some spores, mount the latter in water, examine 

 with a power of about 200 diameters, and draw. 



352. Life History of the Fern. When a fern-spore is sown on 

 damp earth it gradually develops into a minute, flattish object, 

 called a prothallium (Fig. 211). It is a rather tedious process to 

 grow prothallia from spores, and the easiest way to get them for 

 study is to look for them on the earth or on the damp outer surface 

 of the flower-pots in which ferns are growing in a greenhouse. All 

 stages of germination may readily be found in such localities. 



Any prothallia thus obtained for study may be freed from par- 

 ticles of earth by being washed, while held in very small forceps, in 

 a gentle stream of water from a wash-bottle. The student should 

 then mount the prothallium, bottom up, in water in a shallow cell, 



