THE principal objects in the study of plant-life are the organs by means of which they obtain and assimilate substances that are essential for their nourishment and growth, and those by which the perpetuity of their race is maintained and their type transmitted from age to age. In the lowest group of plants, represented by the Algæ, which come first into consideration, the two properties are combined; in the highest they are distinctly different, but the progress from one to the other may be traced through an ascending series of vegetable structure. In the simple grades of vegetables, the primordial cell frequently constitutes the whole plant; it appears first, and then envelopes itself with a coat either of cellulose or of a gelatinous substance. Many instances of this are to be found amongst the Algæ, which are all aquatic plants, and are found growing either attached to other bodies, or floating independently, and live, some species in fresh water, and others in the sea and its estuaries. The Algæ absorb carbonic acid and give out oxygen, under the influence of sun-light, exactly as do the flowering plants; and the quantity of oxygen disengaged by them is said to be enormous. Before proceeding to trace the structure and development of the Algæ, it may be desirable to indicate something of the classification of this curious group of plants. As already stated, they are without exception aquatic plants. They comprise three distinct orders, the Chlorospermeæ, having green spores; the Rhodospermeæ, having red spores; and the Melanospermeæ, having olive-coloured spores. These groups embrace all the varied plants known as sea-weeds, as well as the cellular plants which are developed in fresh water. The Chlorospermeæ are separable into three groups, namely, those which are simply cellular, including the Palmelleæ, the green Desmidiaceæ, and the yellow-brown silicious-coated Diatomaceæ; those which are filamentous, called generally confervas, and including the true Confervaceæ, in which the threads have no compound axis, the Batrachospermeæ, in which the threads are partially incorporated with an axis, the Nostochineæ, in which the slender moniliform threads are invested with a mucous or gelatinous mass, the Oscillatoriæ, and some others; and those which are foliaceous, comprising the Ulvaceæ. All these are monœcious plants, whose reproductive bodies are zoospores provided with ciliary appendages, or motionless cysts filled with endochrome, true spermatozoids being rarely present.