€36 THE SUBDIVISIONS OF THE VEGETABLE KINGDOM. 



life cycle is passed in this fixed condition, and cell divisions take place, the products 

 eventually again developing flagella and being set free as zoospores. The genera 

 Chlorangium and Physocytium are examples of the simplest form of this type of 

 life-history. Forms with a rather more complicated structure in the fixed stage are 

 found in the genera Mischococcus (a common form on the surface of threads of the 

 higher Algae), Euglenopsis (a newly-discovered American plant), and their allies- 

 In these the protoplasm of the zoospore, after fixing itself and putting on a delicate 

 cell-wall, pushes out the surface of its membrane away from the substratum, thus 

 forming a tube of gradually increasing length, the apex of which is always occupied 

 by the protoplasm. Division of the protoplasm and subsequent pushing out of the 

 wall of the tube in difierent directions by the daughter-cells results in a branching 

 of the hollow stalk, and in this way quite a considerable branching plant-body 

 may be produced. Eventually some or all of the cells occupying the apices of the 

 various branches of the tube acquire flagella and escape into the water as zoospores, 

 which again settle on solid objeets and give rise to new plants. 



Other forms in which the cell derived from a zoospore multiplies by division, the 

 products eventually again giving rise to zoospores, are SchizochlaTnys, Botryococcus, 

 BictyosphceriuTn and Tetraspora. In these, however, the immotile phase is not fixed, 

 but forms floating colonies of various conformation. Into this topic we cannot 

 •enter further, except to remark that Tetraspora forms flat colonies of cells arranged 

 in one plane and held together by the swollen mucilaginous cell- walls. CeU division 

 takes place in planes at right angles to that of the colony. This type of colony is 

 specially interesting, as it suggests the form of thallus found in Ulvacece, which in 

 turn appears to lead on to the higher forms Gonfervoidece. 



PleurococcacecB. — More or less closely allied to the above-mentioned genera are 

 ■others which do not form zoospores at all. These types with no motile phase in 

 their life-cycle may be conveniently classed together as Pleurococcacece. The type- 

 ^enus Pleurococcus contains some of the most widely-distributed algal forms known. 

 P. vulgaris forms the bulk of the green coating of damp earth, tree trunks, palings, 

 &c., in all regions of the globe. It consists of roundish cells, dividing in three direc- 

 tions in space and thus forming solid masses of cells hanging together in multiples of 

 two, and often flattened by lateral contact. Each cell contains several parietal chro- 

 matophores which may, however, fuse together to form a single one. Resting aki- 

 netes are formed by the cells ceasing to divide, becoming spherical, and thickening 

 their walls. At the same time oil appears in the protoplasm. It is probably mainly 

 in this phase that Pleurococcus gets distributed by the wind from one place of growth 

 to another. Owing to the resemblance of the akinetes of some of the confervoid 

 Algse to those of Pleurococcus, it has often been stated, and indeed is still held by 

 some algologists that Pleurococcus itself is merely a growth-phase of these higher 

 Algae. But recent culture- experiments leave little room for doubt that Pleurococcus 

 is a perfectly autonomous form, although it may often be associated with pleurococcoid 

 stages of other Algae. Sremosphcera is a pretty form, common in fresh water, with 

 single floating spherical cells. Each cell contains numerous separate chlorophyll- 



