In /Ulva/ there is a double layer of cells. The layers separate in /Enteromorpha/, giving a hollow or tubular form. In /Monostroma/ a double layer is opened or torn apart, giving a frond with a single layer of cells. [pg027] The stem-like forms of certain algæ are composed of cylindrical cells which combine or grow in a longitudinal direction chiefly. Sometimes the cells are arranged evenly, in which case the stem seems articulated, as in some species of /Ceramium/. Again, they are irregularly placed, so that the stem appears solid. The highest types of algæ in the differentiation of parts, or vegetative forms, are to be found in the /Fucaceæ/, of the brown seaweeds; the highest in the reproductive development, in the red class. Reproduction by cell-division, in which the organism itself breaks up into two or more individuals, is called /vegetative reproduction/. Higher forms reproduce by spores, or germ-cells, which give rise to new individuals on germination. The substance of an alga is more or less firm, according as the vegetable mucus or gelatinous matter it contains has more or less consistency; it is /membranaceous/ when the gelatine is scant and glossy, /gelatinous/ when it is abundant and fluid, and /cartilaginous/ when it is hard. Some algæ are annuals; a few are perennials, and cast off and renew their laminæ every season. Many plants present quite a different appearance at different seasons of the year, and so are often difficult to identify. Those which form spores throw off these isolated cells, which sink or are washed to positions where they germinate and begin their cycle of life. Many of the spores begin their growth at once, without regard to season, so the species is ever present. [pg028] VII NAMING OF PLANTS The real or technical names of plants, which at first appear long and unpronounceable, are in reality simple when the system of naming is understood. Every plant has a /generic/ and a /specific/ name. The generic name is analogous to the surname of a person, such as /Smith/ or /Jones/. The specific name is analogous to the Christian name of a person, such as /John/ or /James/. The specific name never stands alone, and would have as little designating character as John —— or James ——. This is called the /binomial/ (two-name) /nomenclature/. It was introduced by Linnæus, and greatly simplified the system of naming. The rule in scientific nomenclature is that all names must be Latin or Latinized. This gives a universal language by which scientists of all countries understand one another. The names of classes (the highest groups) and subclasses are adjectives or adjective nouns, expressing the most prominent characteristic of the class or subclass. Thus the four subclasses of the class /Algæ/ are: /Cyanophyceæ/ (subclass of blue-green algæ). /Chlorophyceæ/ (subclass of grass-green algæ). /Phæophyceæ/ (subclass of dusky-brown or olive-green algæ). /Rhodophyceæ/ or /Florideæ/ (subclass of red algæ).