Chromatophores.The chromatophores or plastids are protoplasmic structures, denser than the cytoplasm, and easily distinguishable from it by their color or greater refractive power.
Penicillus is brush-like, example, there is a marked protoplasmic continuity between all the Halimeda and Cymopolia are jointed, Acetabularia has much the same cells of the colony.
The attachment of the cell of an ooblastema filament to a cell of the thallus may be effected by means of a minute pore, or the two cells may fuse their contents into one protoplasmic mass.
Each is a small protoplasmic body, in the meshes of whose vubstance the green coloring matter chlorophyll is contained in some form of solution.
The conjoined organism is, in fact, a colony or association of the protoplasmic tinits, though each unit retains its independence.
Any one of these soon comes to rest on a host-cell, and either pierces it and empties its contents into its cavity, where the further development occurs (Olpidium), or merely sends in delicate protoplasmic filaments (Rhizophydium) or a short hyphal tube of, at most, two or three cells, which acts as a haustorium, the further development taking place outside the cell-wall of the host (Chytridium).
In the first place, the extremely small size and isolation of the vegetative cells place the protoplasmic contents in peculiarly favourable circumstances for action, and we may safely conclude that, weight for weight and molecule for molecule, the protoplasm of bacteria is brought into contact with the environment at far more points and over a far larger surface than is that of higher organisms, whether - as in plants - it is distributed in thin layers round the sap-vacuoles, or - as in animals - is bathed in fluids brought by special mechanisms to irrigate it.
Tissues.The component parts of the tissues of which plants are composed may consist of but slightly modified cells with copious protoplasmic contents, or of cells which have been modified in various ways to perform their several functions.
In a second type they are situated at the ends of tracheal strands and consist of groups of richly protoplasmic cells belonging to the epidermis (as in the leaves of many ferns), or to the subjacent tissue (the commonest type in flowering plants); in this last case the cells in question are known as epithem.
The sensory cells are slender epithelial cells, often with a cilium or stiff protoplasmic process, and should perhaps be regarded as the only ectoderm-cells which retain the primitive ciliation of the larval ectoderm, otherwise lost in all Hydrozoa.