The cells in most of the Algæ belong to the _parenchymatous_ form; these, however, in the course of their growth, may very often become somewhat oblong; in many Algæ (particularly Fucoideæ and Florideæ) occur, moreover, _hyphæ-like threads_, which are very long, often branched, and are either formed of a single cell, or, more frequently, of a row of cells, having a well-pronounced apical growth. The parenchymatous as well as the hyphæ-like cells may, in the higher Algæ (especially in certain Fucoideæ and Florideæ), be further differentiated, so that they form well-defined anatomico-physiological systems of tissue, _i.e._ assimilating, conducting, storing, and mechanical. With regard to _the external form_, the thallus may present no differentiation, as in many unicellular Algæ, or in multicellular Algæ of the lower order, which are then either equally developed in all directions (_e.g. Pleurococcus_, Fig. 47), or form flat cell-plates (_Merismopedium_) or threads (_Oscillaria_, Fig. 21). The first step in the way of differentiation appears as a difference between apex and base (_Rivularia_, _Porphyra_); but the division of labour may proceed so that differences may arise between vegetative and reproductive cells (_Œdogonium_, Fig. 54); hairs and organs of attachment (rhizoids and haptera), which biologically serve as roots, are developed, and even leaves in certain forms of high order, belonging to different classes (_e.g. Caulerpa_, Fig. 59; _Characeæ_, Fig. 61; _Sargassum_, Fig. 72; and many Florideæ). =The non-sexual reproduction= takes place _vegetatively_, in many instances, simply by division into two, and more or less complete separation of the divisional products (Diatomaceæ, Desmidiaceæ (Fig. 36), many Fission-plants, etc.), or by detached portions of the thallus (_e.g. Caulerpa_, _Ulva lactuca_, etc.; among many Schizophyceæ, small filaments known as _hormogonia_ are set free), or _asexually_ by special reproductive cells (_spores_) set free from the thallus; these may be either _stationary_ or _motile_. The stationary reproductive cells (spores) may either be devoid of cell-wall (tetraspores of the Florideæ), or may possess a cell-wall; in the latter case they may be formed directly from the vegetative cells, generally by the thickening of the walls (_akinetes_), or only after a process of re-juvenescence (_aplanospores_). Aplanospores, as well as akinetes, may either germinate immediately or may become resting-cells, which germinate only after a period of rest.