432 THALLOPHYTES 



substance, called a thattus.* Some beautiful and valuable Dyes are affor Jed by 

 plants belonging to this order. 



*Tbe following definitions, and cbaracteristic details, are from TUCKEKMAN'S 

 valuable Synojisis: 



" Perennial, aerial Algae, vegetating only under the influence of moisture, 

 which is imbibed by the whole surface, propagated by spores (sporidia), and also 

 by the cells (gonidia) of the green layer. 



Thallus (universal receptacle, Ach.) composed of three layers, viz : the cortical, 

 the medullary, and the gonimous ; evolved from a hypothallus (the elementary 

 state in which the layers are confused, and discernible afterwards as cylindrical 

 cells, and also as fibres on the under side of foliaceous Lichenes, and forming the 

 base, closely adnate to the matrix, in crustaceous ones), typically horizontal or 

 vertical. The horizontal thallus is either crustaceous (often somewhat lobed at the 

 circumference or squamulose). or foliaceous (becoming sometimes in degenerate 

 states crustaceous). The vertical thallus is either compressed (sub foliaceous), or 

 terete (fruticulosR) ; of both of which the filamentous thallus and the pendulous 

 thallus are degenerations. In Cladouia and Stereocaulon a vertical thallug 

 (podetium) arises from the primary horizontal thallus, and is itself often be- 

 sprinkled with a kind of secondary horizontal thallus in the form of leaf-like scales. 



Lichenes are reproduced in two ways; 1. by gonidia, the (normally green) cells 

 of the green (gonimous) layer, which appear on the surface as irregularly shaped 

 powdery masses (soredia), and propagate either on the original thallus, forming 

 foliaceous or squamulose expansions, or external to the original thallus, forming 

 new individuals of the parent thallus; and 2. by sporidia, consisting of subglo- 

 bose or elliptical cells, which are either naked or contained in other elongated more 

 or less vertical cells (asci), and immersed in the thalamium (or fructification proper), 

 and propagate new individuals of the species. The thalamium is either rounded, 

 gelatinous-waxy, and the asci converging (nuclei form), or flattened at length into 

 a rigid, persistent, or afterward collapsing lamina (subdisciform), or originally 

 disciform (open) ; and is itself contained in a receptacle (exciple), either of the same 

 color with and like the thallus (thalline exciple), or of different color and nature 

 (proper exciple). The whole fructification constitutes the apothecium, which is 

 typically round, though also occurring normally oblong and linear (lirellaeform,) 

 and is either excavated with a contracted margin (urceolate), or slightly concave 

 with an elevated margin (scutettaeform), or very concave-scutellaeform (cyalhi- 

 form), or very concave-scutellaeform and pervious (infundibuliform, a term 

 applied also to the pervious cup-bearing podetia of Cladoniae), or goblet-shaped 

 and stipitate (crateriform), or dilated, flat, and without prominent margin 

 (peltaeform, of which the reniform is a variation). or convex with repressed mar- 

 gin (cephaloid), or between scutellaeform and peltaeform (disciform), or between 

 gcutellaeform and cephaloid (tuberculate). When the thalline exciple is prolonged 

 below into a footstalk, it is said to be pedicellate ; a proper exciple in like manner 

 prolonged is said to be stipitate. When the proper exciple is originally and typi- 

 cally closed, the apothecium receives the name of perithecium. In the ANGIOCARPI, 

 several thalamia are sometimes contained in the same exciple (composite apothecia) ; 

 and in the GYMNOCARPI, in like manner, several disks are sometimes confluent 

 (symphycarpeous apothecia). The colors of the thallus in Lichens are disposed by 

 I'RIES In four series: 1. from pale green becoming glaucous; 2. from yellowish 

 green becoming ochroleucous ; 3. from dark green becoming fuscous or olivaceous ; 

 4. from pale yellow-green becoming lemon -colored. Each series has its peculiar 

 variation.". The glaucous runs into pale green, ceruleseent, and white; the 

 fuscous into dark green, olivaceous, cinereous, greyish-fuscous, and dark chestnut; 

 the ochroleucous into yellowish green and albescent; the lemon-colored into pale 

 yellow, orange-red, and vermilion-red." 



