1911 Encyclopædia Britannica/Algae/Habit
In many algae certain branches of limited growth bear a remarkable resemblance to leaves. The Characeae among freshwater algae and the Sargassaceae among marine algae might be cited as examples. Surveying the whole range of Habit. algae life, Oltmanns distinguishes bush-forms, whip-forms, net-forms, leaf-forms, sack-forms, dorsi-ventral forms, and cushions, plates and crusts. The similarity of outline in many species to that of trees and shrubs will strike any one who examines algae mounted for the herbarium. Cladophora and Bryopsis among monosiphonous forms, Chara, Polysiphonia, Ceramium and Cystoseira among larger algae, are illustrations of this. The whip-forms are represented by Spirogyra, Chaetomorpha, Scytosiphon, Nemalion, Himanthalia and Chorda. Net-forms are found in Hydrodictyon and Microdictyon. The leaf-forms are very varied and owe their existence to the advantage accruing from the exposure of a large surface to the influence of the light. In some cases such as Delesseria, Neurymenia, Fucus, Alaria, the leaf-like structure is provided with a strengthening mid-rib, and when as in Delesseria it is also richly veined the resemblance to the leaf of a flowering plant is striking. Laminaria, Padina, Cutleria, Punctaria, Iridaea, Ulva, Porphyra, are leaf-like with a rigidity varying from a fleshy lamina to the thin and pliable. Agarum, Claudea and Struvea are leaf-forms which are perforated like Aldrovanda among flowering plants. Enteromorpha, Asperococcus and Adenocystis are sack-forms. Dorsi-ventral algae are rare. Leveillea jungermanneoides bears a remarkable resemblance to a leafy liverwort. In the next group of forms the simplest are crusts attached to the substratum throughout their extent, and growing at the margin. Such are Myrionema, Ralfsia, Melobesia and Hilderbrandia. Others are attached throughout their extent, but also grow vertical filaments so as to form a velvety pile. Such are Coleochaete, Ochlochaete, Elachistea, Ascocyclus and Rhododermis. Peysonellia squamaria, Melobesia lichenoides, Leathesia difformis are forms which are not attached throughout but grow in plates like the foliaceous lichens.