10 CONTRIBUTIONS FROM THE NATIONAL HERBARIUM. 
(fig. 1, d, p. 9), Rivularia (fig. 2, a), Polycoccus (fig. 2, 6), Dactyl- 
ococcus (fig. 2, ¢), and Sirosiphon (fig. 2, e). 
GROSS MORPHOLOGY. 
Under this head we shall consider only those lichen structures 
which may be seen readily with the unaided eye or with an ordinary 
hand lens. 
THE THALLUS., 
The fundamental part of a lichen is its vegetative tract or thallus. 
The thallus may be an erect structure, rising from the substratum, a 
pendulous one hanging downward from it, a conspicuous or in- 
conspicuous flat one closely or loosely attached to the substratum, or 
== —_ eo secee 
Fig. 2.—Algal types with associated fungal hyphe. a, Rivularia with the fungal hyphe intermingled. 
b, Polycoccus: colonies of cells surrounded by hyph and haustoria. c, Dactylococcus: elliptical algal 
cells and fungal hyphae. d, Pleurococcus, the irregular algal cells inclosed in fungal hyphe. ¢, Siro- 
siphon, a terminal branch not showing the fungal hyphe. a, Much enlarged; b,c, enlarged 325 diam- 
eters; d, 450 diameters; e, 250 diameters. From Schneider. 
an inconspicuous one largely or even wholly embedded in the sub- 
stratum. Erect and pendulous forms are commonly called fruticose 
thalli (pl. 43, A, facing p.210), and the flat or horizontal ones may be 
either foliose (pl. 28, facing p. 157) or crustose (pl. 8, B, facing p. 100) ; 
foliose when somewhat leaf-like and crustose when forming a closely 
attached crust resting on or within the substratum. Various inter- 
mediate conditions between crustose and foliose and between foliose 
and fruticose types of thalli may be expected in the study of lichen 
species. 
GENERAL FORMS OF THALLI. 
Of the foliose forms, many are variously lobed (pl. 30, facing p. 160) 
and some are quite entire at the margin. In instances where the lobing 
is evident the lobes may be more or less imbricated. In both lobed 
and unlobed forms the margin may be wavy or crenate instead of 
