CABPOPHTTA. 169 



place by means of minute cells (sperm-cells, or spermatid), 

 ■which are produced in countless numbers in cavities {sper- 

 mogones) in the lichen-body. The sperm-cells come in 

 contact with the projecting filament {trichogyne) , doubtless 

 by means of mnds, the result of which is the rapid upward 

 growth of filaments which ultimately produce spore-sacs 

 and spores in discs, as above described. 



Practical Studies. — (a) Collect fruiting specimens of the common 

 fruticose lichen shown in Fig. 90, B, whicli grows upon branches of 

 trees in forests. Make tliin cross-sections of the stem, mount in alco- 

 hol, afterwards adding dilute potassic hydrate. Study the filaments, 

 and their relation to the gonida. Isolate some of ihe gonidia by 

 tapping on the cover-glass, and note their resemblance to Green 

 Slime. 



(J) Make thin vertical sections through one of tlie fruiting discs, 

 mount as above, and study spore-sacs, spores, and paraphyses. 



(c) Collect some of the small, flat, many-lobed lichens which grow 

 on the bark of apple-, maple-, and oak-trees, and having small blackish 

 fruit-discs. Make careful sections of the plant-body through the 

 fruit-discs, and study the whole structure, spores, spore-sacs, para- 

 physes, filaments, and gonidia. (Compare with Fig. 93.) Here also 

 the gonidia closely resemble Green Slime. 



354. The Rusts {Order TTredinece) are minute parasitic 

 plants, which grow in the tissues of higher plants. Their 

 life-history is only imperfectly known; nothing as yet being 

 known as to their sexual organs, if indeed they have any. 



355. The common Wheat-rust (Puccinia graminis) may 

 be taken as an illustration of the order. It is common 

 wherever wheat is grown, and often greatly injures and 

 sometimes entirely destroys the crop. Its round of life 

 shows four well-marked stages, as follows: (I) In the spring 

 clusters of minute yellowish cups break through the tissues 

 of the leaves of the Barberry. These cups are at first 

 rounded masses of conidia which develop on the internal 



