648 THE SUBDIVISIONS OF THE VEGETABLE KINGDOM. 



A whole series of fossil forms from the chalk and tertiary deposits serve to con- 

 aect the various existing types of these and allied genera. 



Alliance VIII. — Confervoidese. 



The Algae included under this alliance possess a type of thallus composed of dis- 

 tinct and separate cells. These cells are united usually into linear series, which form 

 branched or unbranched threads. In a few families, however, cell-division takes 

 place in two, or even three, dimensions, resembling some Protococcoidese in the forma- 

 tion of cell-surfaces or cell-masses. Zoospores are produced by nearly all conf ervoid 

 forms. Aplanospores and akinetes are common. The gametes may be isogamous, 

 or they may show marked sexual diiferentiation. 



Families: Ulvacece, Ulotrichacece, Cylindrocapsacece, (Edogoniacece, Clado- 

 phoraceoe, Gomontiacece, SphceropleacecB, Chcetophoracece, Trentepohliaceoe, Mycoi- 

 deacecB, Coleochcetacece. 



Ulvacece. — This family is usually regarded as the lowest of the confervoid series. 

 It is characterized especially by forming cell -surfaces instead of filaments. Zoo- 

 spores with four flagella and isogametes with two, as well as akinetes, are 

 formed in the group. Monostroma very much resembles Tetraspora among the 

 Protococcoideaa, from which we may suppose the confervoid forms to have arisen 

 in evolution. The thallus consists of a single layer of roundish or angular cells. In 

 germination, the zygote divides to form a small hollow sphere, which splits, and 

 extends itself to form a flat plate. At first fixed by rhizoids, the thallus later floats 

 freely in the water. The chromatophore is a parietal plate, covering more or less of 

 the cell-wall, and contains a single pyrenoid. Gametes (which may develop without 

 conjugation), or zoospores may be formed in almost any cell of the thallus. 



TJlva differs from Monostroma in possessing a thallus of two layers of cells, 

 those of each layer dividing independently of the other. The zygote germinates to 

 form a fixed cell thread, which later on produces the two-layered thallus. Ulva 

 latissima (the Green Laver or Sea-lettuce) is very common on the rocks of our 

 coasts near high-tide mark. It forms large green wavy fronds firmly fixed to the 

 substratum. It is sometimes used as an article of food. 



JEnteromorpha is a large genus, several species of which are common on our 

 coasts, and some in fresh water. The thallus forms a branched hollow tube, the 

 wall of which is one cell thick. Any cell of the thallus may act as the apical cell 

 of a branch. The apex of the branch is solid, but the cells soon round themselves 

 off" to form the wall of the tube. The gametes and zoospores resemble those of 

 Monostroma and Ulva. 



Letterstedtia is a Cape and Australian form much like Ulva, but with a deeply- 

 lobed thallus. Zoospores are produced only in the cells of the lobes. 



Ulotrichacece. — This family contains several genera common in fresh water, and 

 some marine forms. The thallus consists of an unbranched filament of cells seldom 

 much longer than they are broad. The chromatophore is single, parietal, and of 



