Their structure is eminently that of degenerate forms. Many frequent growths of coralline Algae and hydroid polyps, upon the juices of which they feed, and in some cases a species of gall is produced in hydroids by the penetration of the larval Pantopod into the tissues of the polyp.
The rapidity with which walls, piles and pontoons - stone, wood and iron - become covered with marine plants is well known, while the discovery of some effective means of preventing the fouling of the bottoms of ships by the growth of algae would be hailed as a boon by shipowners.
When it is sought to consider algae with a view to the correlation of the external form to the conditions of life, a subject the study of which under the name of ecology has Ecoiogy.
The attachment organ of algae is thus more properly called a holdfast, and is found to be of very varied structure.
Again, many Green Algae - some unicellular, like Sphaerella and Chlamydomonas; some colonial forms, like Volvox and Hormotila: some even filamentous forms, like Ulothrix and Stigeoclonium- are known to pass into a condition resembling that of a Palmella, and might escape identification on this account.
The Nuclei of the Lower Plants.It is only in comparatively recent times that it has been possible to determine with any degree of certainty that the minute deeply stainable bodies described more especially by Schmitz (1879) in many Algae and Fungi could be regarded as true nuclei.
Without this occasional drying of the soil the finer grasses and the leguminous plants will infallibly be lost; while a scum of confervae and other algae will collect upon the surface and choke the higher forms of vegetation.
Similar modes of growth occur among the Siphoneous Green Algae and also among the Red Seaweeds.
It is also possible in the absence of satisfactory intermediate forms that the Ascomycetes and Basidiomycetes have also been derived from the algae independently of the Phycomycetes, and perhaps of one another.
The occurrence of characteristic algae at different levels constituting the zones to which reference has already been made, is probably in part an expression of the fact that different species vary in the capacity to resist desiccation from exposure.