THALLOPHYTA. 637 



bodies, embedded in a parietal layer of protoplasm, and a nucleus suspended by 

 protoplasmic strands in the centre of the cell. Multiplication is effected by division 

 of the protoplasm into two daughter-cells which escape by rupture of the mother- 

 cell membrane. Scenedesmus is another motionless floating fresh-water form. It 

 consists of oblong cells united into groups of two, four, or eight, which lie side by 

 side, palisade fashion. Some or all of the cells often possess straight or horn-like 

 projections of their walls, which give the cell groups a very characteristic appear- 

 ance. The single solid chromatophore occupies nearly the whole cell cavity. Ghlo- 

 rella is a genus whose cells are symbiotic with Radiolaria (yellow cells). Other 

 forms live in a similar relation with certain Coelenterates and Platyhelminths. 

 Several help to form lichens. Since the various genera of Pleurococcacece differ 

 thus very widely in the form and structure of their cells, and indeed are only united 

 by the negative character of the absence of zoospores, it is almost certain that they 

 cannot be considered as forming a natural group. The various genera are very 

 probably allied to different neighbouring groups from which they have been derived 

 by the suppression of the habit of forming zoospores. 



The Undosphceraceoe are a small and very natural group of unicellular Algse, 

 characterized by their habit of living in the intercellular spaces of various higher 

 plants. They possess motile zoospores, or gametes, or both, but the motionless cells 

 produced from these do not undergo vegetative divisions. Very possibly they 

 represent a separate line of descent from the Chlamydomonadese, a line of descent 

 in which the motionless cell has become the dominant phase in the life-cycle, and 

 has been specially adapted to the new conditions of life, but differs from the motion- 

 less cells of the " Tetrasporaceas " in directly forming zoospores without undergoing 

 purely vegetative divisions. 



Two forms of Endosphseraceae may be taken as illustrations of this type of life- 

 history. 



ChlorochytriuTn Lemnce inhabits the intercellular spaces immediately under the 

 epidermis of the leaves of Lemna trisulca (the Ivy-leaved Duckweed). Each plant 

 consists of a single, thick- walled, oval cell with a parietal chromatophore containing 

 numerous pyrenoids and a large central vacuole. Very numerous pear-shaped 

 isogametes are formed by successive divisions of the protoplasm of the cell. Then 

 a layer of substance outside the mass of gametes (probably the ectoplasm of the 

 cell) begins to swell strongly, and bursts not only the cell- wall but also the super- 

 incumbent tissue of the Duckweed leaf, forming a sphere of mucilage in which the 

 gametes begin to swarm and to conjugate in pairs. Spherical zygozoospores are 

 thus produced; these escape from the mucilage, and after some free swarming in 

 the surrounding water, settle on the boundary between two epidermal cells of a 

 Duckweed leaf, draw in their flagella, put on a cell-membrane, and form a definite 

 parietal chlorophyll-body with a single pyrenoid. After two or three days a delicate, 

 colourless tube is put out, which forces its way between the two epidermal cells of 

 the leaf, and reaches an intercellular space. The contents of the zygote slowly pass 

 over into the apex of this tube, which gradually increases in size and assumes the 



