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plant-body is termed the thallus (Th., Fig. 2). The thallus increases in size 
by elongation and repeated branching of the lobes near their tips : hence the 
older portions are towards the center. From the under side of the thallus 
are developed numerous projections, called rhizoids (Rz., Fig. 2), which 
serve to attach the plant to the “substrate,” or surface upon which it rests. 
Occasionally, upon the upper surface of the thallus appear little granular or 
powdery heaps (Sd., Fig. 2), called soredia. These are sometimes so 
numerous as to alter considerably the appearance of the lichen. Finally, 
close scrutiny with the magnifier will bring to view a number of black specks 
scattered irregularly over the lobes. Each is the mouth of a small cavity, 
called a spermagone , which extends into the body of the thallus (Sg., Fig. 2). 
If a very thin slice cut across the thallus be examined with a magnifying 
power of two or three hundred diameters, the structures shown in Fig. 3 are 
exhibited. The principal 
mass is composed of deli- 
c a t e tubular threads, 
called hyphce (H), which 
are rather loosely inter- 
woven to form an inner 
or medullary layer (M), 
and firmly compacted 
toward the surface of the 
thallus, forming a cortex 
(C) above and below. 
Just below the upper cor- 
tex is an irregular layer composed of innumerable bright green bodies (G) 
interspersed among the medullary hyphse. These little bodies are termed 
gonidia. In each we may distinguish a transparent envelope surrounding 
the green protoplasmic contents. Close examination will show that branches 
of the medullary filaments are often in intimate contact with the gonidia. 
For many years after their discovery it was believed that this close con- 
nection indicated that the gonidia were outgrowths from the hyphae or vice 
versa. In 1869, however, the great German botanist Schwendener showed 
there were strong reasons for believing that the gonidia are not genetically 
connected with the hyphse, but are minute Algae , upon and around which 
had grown the hyphse of a parasitic fungus; in other words, that a lichen 
is not a single .individual possessing as organs hyphse and gonidia, but 
is a community consisting of (i)a host of small Algae — such as one finds 
growing by themselves on trees and rocks — and (2) a fungus, the like of 
which is also found living separately upon bark, but which in a lichen has 
become adapted to imprison Algae and gain nutriment from them. 
Ever since Schwendener’s time important evidence has been accumu- 
lating to confirm his view, until to-day it appears to be as well proved as 
Harvey’s theory of the circulation of the blood. Without going at length 
into the details of this evidence, we may cite, in brief, the following facts: 
1. All known forms of gonidia have been found to resemble species of 
Algae (belonging to several diverse families) which grow in situation 
Fig. 2. 
The same. Digrammatic vertical section. Th., 
thallus; Rz., rhizoids: Sd., sorendia: Ap., apo- 
thecium: Sg., spermagone. (Original.) 
