AND LIFE-HISTORY OF A TROPICAL EPIPHYLLOUS LICHEN. 91 
at the affected area, the rhizoids described above may be distinctly seen creeping on the 
surface of the substratum, and more or less compressed between it and the thallus whence 
they arise (cf. Pl. XVIII. figs. 8,9, © Pl. XIX. figs. 12, 11). Since, as the examination of 
the under side of the thallus ez face proves, these rhizoids take a sinuous course, they 
appear eut in all kinds of sections, transverse and oblique to their long axes. In some 
cases (fig. 12) loosely creeping on the cuticle of the leaf, in others they are more or less 
intimately blended with or even sunk into its substance (fig. 8), as if they had partially 
eaten a way in. 
In a young thallus, the outer surface generally presents an unbroken outline; but at 
an early date in the process of development, the outer or upper walls of certain cells 
become bulged out, and grow forth as tubular outgrowths. These, like the rhizoids 
above described, usually became cut into several successive chambers by transverse 
septa; but instead of remaining thin-walled, and describing the sinuous courses of the 
rhizoids, they project stiffly from the edges and surface of the thallus, and form short or 
long, simple or multicellular hairs. Some remain parallel to the leaf-surface, and, except 
that their walls are thicker and their long axes straight, resemble the rhizoids; the 
others stand forth free into the air (cf. Pl. XVIII. figs. 5, 7; Pl. XIX. figs. 15, 16, &c.), 
at various angles from the surface of the leaf. 
The marginal ** hairs " commonly remain short and simple, or Leonie divided by one 
or two cross walls into two or three chambers (fig. 5): the walls are smooth, and somewhat 
thick and firm, especially at the bluntly produced apex. The contents of their cells are, 
as before, granules and orange-red fatty matters, enclosed in a protoplasmic basis tinged 
more or less with pale apple-green. Under certain conditions the red matter is in abeyance, 
and the green colour predominates. Ав before, and as in the other hairs described below, 
every cell is usually abundantly supplied with starchy contents, striking a deep indigo- 
blue with iodine. 
Those hairs which stand stiffly upright are longer, with few chambers and cross walls, 
and generally very brightly coloured by the orange-red oil-and-starch-containing sub- 
stances; they are chiefly of two kinds, differing in structure and functions. First 
may be mentioned pointed, stiff, but slender outgrowths, differing but little, except in 
length, from the marginal hairs already described (figs. 5 & 16); second, equally long or 
longer, ascending, multicellular hairs, with the basal walls much thickened, and the apex 
developing into a swollen body, whence originate several radiating, short, tubular branches, 
each of which eventually produces an oval body, the contents of which pass over into 
zoospores (Pl. ХІХ. figs. 15, 21, & Pl. XX. fig. 24). І may, in order to avoid introducing 
any special terms for these bodies and the trichomes producing them, speak of the two 
kinds of hairs just described as simply “ barren hairs” and “ fertile hairs” respectively. 
Before concluding this account df the purely vegetative system of the Alga, it may be 
well to describe the manner of growth of the thallus from its early stages. The cellular 
plate forming this originates as a spherical cell of the simplest description—a zoospore, 
in fact, which has come to rest on the bedewed surface of the leaf. This sphere is at 
first pale apple-green, and possesses a very delicate limiting membrane. In the pale- 
green matrix may be many or few orange-red oily drops, or granules ; and starchy matter 
appears to be always present. In some cases, at least (Pl. XX. fig. 37), a scarcely per- 
