CARPOPHYTA. 169 
place by means of minute cells (sperm-cells, or spermatia), 
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 winds, 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, which grows upon branches of 
trees in forests. Make thin 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 the gonidia by 
tapping on the cover-glass, and note their resemblance to Green 
Slime. 
(0) Make thin vertical sections through one of the 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. 92.) Here also 
the gonidia closely resemble Green Slime. 
354. The Rusts (Order Uredinew) 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 
