Botrydium, a simple siphonaceous alga with root-like attachment.
The group has until recent years been regarded as comprising three classes distinguished by well-marked physiological featuresthe Algae (including the Seaweeds) which contain chlorophyll, the Fungi which have no chlorophyll and therefore lead a saprophytic or parasitic mode of life, and the Lichens which are composite organisms consisting of an alga and a fungus living together in a mutual parasitism (symbiosis); Bacteria were regarded as a section of Fungi.
The relations of the Alga and the Fungus, which have formed a close associationship in the structure known as the Lichen, were established many years ago.
The alga by its own peculiar movement will soon form a radiating circle, perfectly free from dirt, around the coin, which may then be removed.
On a bare rocky surface a fungus would die from want of organic substance and an alga from drought and want of mineral substances.
An alga infested with a Chytridiumindeed, matters can hardly be otherwise.
E, Vertical section of frond of the complicated Siphoneous Green Alga Halimeda.
Brown or even blood-red stripes have been observed in the North Atlantic when swarms of the copepod Calanus finmarchicus were present; the brown alga Trichodesmium erythraeum, as its name suggests, can change the blue of the tropical seas to red; swarms of diatoms may produce olive-green patches in the ocean, while some other forms of minute life have at times been observed to give the colour of milk to large stretches of the ocean surface.
In the case of Scenedesmus acutus it is said that the alga is unable to take up nitrogen in the form of a nitrate or ammoniacal salt, and requires some such substance as an amide or a peptone.
Filamentous diatoms may be mounted like ordinary seaweeds, and, as well as all parasitic algae, should whenever possible be allowed to remain attached to a portion of the alga on which they grow, some species being almost always found found parasitical on particular plants.