Algae

Algae, a class of plants of which the best-known are the seaweeds, though there are fresh-water representatives of almost every subdivision of the class. Like ferns, mosses, and fungi they do not produce true flowers or seeds, and are, therefore, termed Cryptogamia (q.v.); but, like fungi and unlike ferns and mosses, they present no true distinction of stem or axis and leaf or lateral appendage, the whole of their structure being cellular, i.e. without any vessels. Algae and fungi are, therefore, united as the sub-kingdom Thallophyta (q.v.). Many of the larger Algae have cylindric stem - like stalks, structures called "rhizoids," resembling roots, and flattened leaf-like fronds; but these fronds are commonly terminal, not lateral, and there is no distinction in internal structure, whilst the rhizoids are mere organs of attachment, not of food-absorption Though entirely cellular, some Algae have a thickened epidermal or pseudo-cortical layer externally, and the kelp-weed group (Laminarieae) have a zone of tissue (meristem) in which growth by cell-division occurs, thus increasing their diameter much as do some of the higher plants. The Algae differ from Fungi in containing the green colouring-matter chlorophyll (q.v.), common to so many groups of plants. To take this as a fundamental distinction seems objectionable, as being a physiological rather than a structural character, and accordingly in 1874 Sachs endeavoured to substitute four structural grades, Protophyta, Zygosporeae, Oosporeae and Carposporeae (each including both algal and fungal forms) based upon the methods of reproduction; but the older division is now adopted as more natural. The class Algae may thus be briefly defined as thallophytic cryptogams containing chlorophyll. Living almost exclusively in water, either salt, brackish, or fresh, or in damp places, Algae have also been termed Hydrophyta. In structure they present every grade, from a single cell to a filament of elongated cells end to end (monosiphonous), several parallel filaments (polysiphonous), or the large pseudostems and leafy fronds already mentioned. Reproduction is effected by simple cell-division; by the formation of free-swimming ciliated bodies called "zoospores," or of motionless structures produced four together in a fructification or "sporangium" and hence termed "tetraspores;" or by sexual "oospheres" or egg-cells, fertilised by motile ciliated "antherozoids." Some Algae secrete much carbonate of lime, the Corallines being entirely covered with it, and the microscopic Diatomaceae form silicious skeletons with geometrical markings of great beauty. The chlorophyll is frequently accompanied by other colouring matters, the blue phycocyan, the brown phycophaein and the red phycoerythrin, and these afford an obvious distinction between four sub-classes which have also structural characters. These are the unicellular Cyanophyceae or blue-green Algae, including Chroococcaceae, Nostocaceae, Oscillatorieae and Seytonemeae; the Chlorophyccae, or green Algae, mostly in fresh or shallow water, the resting cells of which often turn red, as in the Red Snow plant, their chlorophyll being reduced to chlororufin, including Siphoneae, Volvocineae (the "globe animalcules"), Protococcaceae, Confervoideae, Conjugatae, Desmidiaccae and Diatomaoeae; the Phaeophyceae, Melanophyceae, or olive-brown seaweeds! all marine, mostly between tide - marks, including the kelp-weeds, Laminarieae, and the bladder-wracks, Fucaceae and the Rhodophyceae, Florideae, or red Algae, mostly from deeper water, including the Corallines.