The Macrocystis pyrifera, an Algæ, is the largest of all known plants. It is a sea weed that floats free and unattached in the ocean. Covers the area of two square miles, and is 300 feet in depth (Reinsch). At the same time its structure on examination shows it to belong to the same class of plants as the minute palmellæ which we have been studying. Algæ are found everywhere in streams, ditches, ponds, even the smallest accumulations of water standing for any time in the open air, and commonly on walls or the ground, in all permanently damp situations. They are peculiarly interesting in regard to morphological conditions alone, as their great variety of conditions of organization are all variations, as it were, on the theme of the simple vegetable cell produced by change of form, number, and arrangement. The Algæ comprehend a vast variety of plants, exhibiting a wonderful multiplicity of forms, colors, sizes, and degrees of complexity of structure, but algologists consider them to belong to three orders: 1. Red spored Algæ, called Rhodosporeæ or florideæ. 2. The dark or black spored Algæ, or Melanosporeæ or Fucoideæ. 3. The green spored Algæ, or Chlorosporeæ or Confervoideæ. The first two classes embrace the sea-weeds. The third class, marine and aquatic plants, most of which when viewed singly are microscopic. Of course some naturalists do not agree to these views. It is with order three, Confervoideæ, that we are interested. These are plants growing in sea or fresh water, or on damp surfaces, with a filamentous, or more rarely a leaf-like pulverulent or gelatinous thallus; the last two forms essentially microscopic. Consisting frequently of definitely arranged groups of distinct cells, either of ordinary structure or with their membrane silicified--Diatomaceæ. We note three forms of fructification: 1. Resting spores produced after fertilization either by conjugation or impregnation. 2. Spermatozoids. 3. Zeospores; 2, 4, or multiciliated active automobile cells--gonidia--discharged from the mother cells or plants without impregnation, and germinating directly. There is also another increase by cell division. SYNOPSIS OF THE FAMILIES. 1. _Lemaneæ_.--Frond filamentous, inarticulate, cartilaginous, leathery, hollow, furnished at irregular distances with whorls or warts, or necklace shaped. Fructification: tufted, simple or branched, necklace shaped filaments attached to the inner surface of the tubular frond, and finally breaking up into elliptical spores. Aquatic. 2. _Batrachospermeæ_--Plants filamentous, articulated, invested with gelatine. Frond composed of aggregated, articulated, longitudinal cells, whorled at intervals with short, horizontal, cylindrical or beaded, jointed ramuli. Fructification: ovate spores and tufts of antheridial cells attached to the lateral ramuli, which consist of minute, radiating, dichotomous beaded filaments. Aquatic.