6 THALLOGENS. 



ture ; 3rd, we must regard all the forms of lower Algse as vegetations of a 

 very simple stnictm-e, and distinguish them from each other, notwithstand- 

 ing that in certain circiunstances they may raise themselves to vegetations 

 of a higher form ; for in other circumstances they can exist and multiply 

 independently ; 4th, the same superior fomiation may be produced by 

 primitive formations of altogether different kinds." 



It is not easy to settle the limits of the alliances of Thallogens. Linnaeus 

 and Jussieu had but two divisions, viz., Algse (including Lichens) and 

 Fungi ; and they have been followed by some modern botanists, particularly 

 Fries and Wahlenberg. Others have been satisfied with separating the 

 Lichens from Algse, which, indeed, was virtually done by most of those 

 who acknowledged but two divisions ; and mth admitting three equally 

 distinct groups. Some, on the contrary, have sought to multiply the orders, 

 as De Candolle and others, by introducing a tribe called Hypoxyla ; 

 Greville by adopting the latter, Gastromyci, Byssoidese, and Epiph}i;8e, and 

 proposing a new group under the name of Chastophoroidese ; and finally, 

 Adolphe Brongniart, who carries the number of groups in this di\dsion of 

 Acotyledones as far as 12, viz. Lichens, Hypoxyla, Fimgi, Lycoperdacese, 

 Mucedineffi, Uredinese, Fucaceae, Ulvacese, Ceramiaceae, Confervae, Chao- 

 dinese, and Arthrodieae ; part of which have originated with himself, and 

 others with Bory de St. Vincent. It is clear, however, that these groups 

 are of very unequal degrees of importance, and that after aU they must be 

 reduced under the three great forms whose existence is universally 

 recognised. 



In what way those forms can be best defined is a very difficidt question. 

 It has been said that Algae are aquatics, while Lichens and Fungi are 

 terrestrial ; but Fungi will develop in water, when they assume the form of 

 Algae. Lichens have been characterised by their shields, or reproductive 

 disks containing spores lying in the fusiform spore-cases caUed asci ; but a 

 whole division of Lichens consist of genera without such asci. Then as to 

 Fungi, they have been characterised by the want of a thallus, which is 

 essential to Lichens ; but the mycelium or spawn of Fungi is reaUy a 

 thallus ; and it is impossible to distinguish by that character the genus 

 Verrucaria of Lichens from Sphasria of Fungi. According to two of the 

 most skilful of our modern systematists, the following are the distinctions of 

 the three great groups : — 



Agardh (1821), 



1. AhQjE. Aquatic plants, filamentous, lamelliform, or leafy, intensely and brightly coloured, 



including spores, which are either contained in pericarps or scattered over the surface. 



2. FuxVGi. Fugacious, pulverulent, flocculent, cnistaceous or fleshy plants, arising out of the 



destruction of organic matter (or capable of doing so), whitish, or coloui-ed, not green, 

 with their spores immersed. 



3. Lichens. Perennial plants, crustaceous, laminated or filiform, not of a leaf-green, including 



spores plunged in a thallus as well as in shields. 



AooLPHB Brongniart (1843). 



1. Alg-e. Frond cellular, living in fresh or salt water (rai-ely in very moist air), fixed by suckers 



or little roots. 



2. Fungi. ThaUus filamentous (or Mycelium), developed on land or in dead or li\-ing organi- 



bodies, producing reproductive organs externally. 



3. Lichens. Frond of various forms, liAing in air, fixed by ceUular fibrils, without a thallus 



developed in subjacent bodies. Fructification, occupying limited spaces on the surface 

 of the frond, fonned of thecae mixed with paraphyses. 



Neither of these definitions is however satisfactory ; they hold indeed in 

 many cases ; but many Fungi have not a filamentous thallus ; again some 

 Lichens (especially if CoUema be included) have a filamentous thallus, and 

 some species are all but aquatic, e. (/. YeiTucaria submersa. In Algals again, 



