THALLOGEN-*;. 723 



Tuber, the Truffle. — The species of Trjffle, several of which occur in 

 Britain, are subten-anean. They are highly esteemed as seasoning or flavour- 

 ing agents. The best are imported from France, Algeria, and Italy ; they 

 are commonly preserved in oil. T. cestivum, T. cibarium, T. melanosporum 

 are the more frequently used species. 



Uredo. — The species of this genus produce the diseases of Com and other 

 cultivated plants, called blights, rusts, &c. 



Natural Order 295. Alg.s;. — The Sea-weed Order. — Diagnosis. 

 — Parenchymatous cellular plants ; growing in salt or fresh 

 water, or in moist situations. The thallus is foliaceous and 

 branched {fig. 7), or filamentous {figs. 831 and 839), or pul- 

 verulent. Many Algse are microscopic, and others are of large 

 .size. In colour they are usually greenish, rose-coloured, or 

 brown. They are reproduced in various ways. (See pp. 

 380—385.) 



Division of the Order, and Examples of the Genera: — The 

 order is commonly divided into three sub-orders, which are fre- 

 quently regarded as distinct natin-al orders ; these are known under 

 the names of tlie Melanosporea, Mekmos2)er/nca',OT Fucoidca ; the 

 likodosporecs, Bhodos2xr7ne(P, ov FloridecB; and the ChlorosporecB, 

 CMorospermecB , or ConfervoidccB. To these siib-orders may be 

 added two others, called respectively Diatomacea and Volvo- 

 cinecB. The distinctive characters of these different sub-orders 

 may be briefly described as follows: — 



Sub-order 1. Melanosporca , Melanospermece, Fucoidecs, or Brown- 

 coloured Alga. — Multicellular Algae, growing in salt water, 

 forming a foliaceous {fig. 7) or filamentous thallus {fig. 839), 

 and of an olive-green or olive-brown colour. Eeproduced by, 

 1. Zoospores {fig. 839); 2. Spores {figs. 7, t, and 8-10); and, 

 3. Antkeridia {figs. 841 and 842). (See pp. 383—385.) 

 Eaccoiiples : — Sargassum, Fucus, Ectocarpus. 



Sub-order 2. Rhodosporea, Rhodosperritecs, Floridcce, or Rose- 

 coloured AlgcB. — Marine multicellular plants, with a foliaceous 

 or branched filamentous thalliis, and of a reddish-purple, rose- 

 coloured, or reddish-brown colour. Reproduced by, 1 . Tetra- 

 spores{figs. 834-836) ; 2. Spores {figs. 837 and 838); and, 3. 

 Antheridia. (See pp. 382 and 383.) Exa/nples : — Corallina, 

 Chondrus, Porphyra. 



Sub-order 3. Chlorosporece, Chlorospcrnitce, Confer voide(e, or 

 Green-coloured Alg(B. — Unicellular or multicellular Algse, grow- 

 ing in fresh or salt water, or in moist situations ; usually of a 

 bright green colour, or rarely red. Reproduced by, 1. Spores, 

 formed either by conjugation {fig. 831) or by impregnation 

 from sjpermatozoids ; and, 2. Zoospores {fig. 832). (See pp. 

 380 — 382.) Examples: — Caulerpa, Palmella, Zygnema. 



Sub -order 4. DiatomaccG. — Brittleworts. — The following dia- 

 gnosis is faom Henfrey : — " Microscopic unicellular plant.s, 

 occurring isolated or in groups of definite f<jrm, usually sur- 



