If the plants are gathered while wet, and packed so as to prevent evaporation of the water, they will keep perfectly for several days, and may readily be shipped for long distances. If they are to be studied away from the seashore, sections for microscopic examination should be mounted in salt water (about 3 parts in weight of common salt to 100 of water). If fresh material is not to be had, dried specimens or alcoholic material will answer pretty well. To study the minute structure of the plant, make a thin cross-section, and mount in salt water. The inner part or pith is composed of loosely arranged, elongated cells, placed end to end, and forming an irregular network, the large spaces between filled with the mucilaginous substance derived from the altered outer walls of these cells. This mucilage is hard when dry, but swells up enormously in water, especially fresh water. The cells grow smaller and more compact toward the outside of the section, until there are no spaces of any size between those of the outside or rind. The cells contain small chloroplasts like those of the higher plants, but owing to the presence of the brown pigment found in all of the class, in addition to the chlorophyll, they appear golden brown instead of green. No non-sexual reproductive bodies are known in the rock weeds, beyond small branches that occur in clusters on the margins of the main branches, and probably become detached, forming new plants. In some of the lower forms, however, _e.g._ _Ectocarpus_ and _Laminaria_ (Fig. 28, _A_, _C_), zoöspores are formed. The sexual organs of the rock weed, as we have already seen, are borne in special cavities (conceptacles) in the enlarged ends of some of the branches. In the species here figured, _F. vesiculosus_, the antheridia and oögonia are borne on separate plants; but in others, _e.g._ _F. platycarpus_, they are both in the same conceptacle. The walls of the conceptacle (Fig. 26, _B_) are composed of closely interwoven filaments, from which grow inward numerous hairs, filling up the space within, and often extending out through the opening at the top. The reproductive bodies arise from the base of these hairs. The oögonia (Fig. 26, _C_, _E_) arise as nearly colorless cells, that early become divided into two cells, a short basal cell or stalk and a larger terminal one, the oögonium proper. The latter enlarges rapidly, and its contents divide into eight parts. The division is at first indicated by a division of the central portion, which includes the nucleus, and is colored brown, into two, four, and finally eight parts, after which walls are formed between these. The brown color spreads until the whole oögonium is of a nearly uniform olive-brown tint.