In this article we will discuss about the classification of phaeophyta.
Kylin (1933) divided Phaeophyta into three classes according to the type of life history. Further division of these classes into subclasses, and the arrangement of the orders, may best be understood from the key given below. This classification has received wide recognition for practical purposes although Papenfuss (1951) rejected the use of the classes and subclasses on phylogenetic grounds.
1. Plants with an alternation of generations
1. Plants without an alternation of generations
2. With an alternation of macroscopic, usually isomorphic generations
Orders: Ectocarpales, Sphacelariales, Cutleriales, Tilopteridales, Dictyotales
2. With an alternation of heteromorphic generations
3. Thallus composed of cells which adhere to one another to form a pseudoparenchymatous tissue
Orders: Chordariales, Sporochnales, Desmarestiales
3. Thallus composed of cells dividing by intercalary longitidinal walls to form a true parenchymatous tissue
Orders: Dictyosiphonales, Laminariales
The Phaeophyta, as viewed by many phycologists, include a single class Phaeo- phyceae which again can be divided into eleven orders details of which are incorporated below:
I. Order Ectocarpales:
Thallus pseudoparenchymatous or true parenchymatous type or of branched filaments; growth usually intercalary, typical meristematic apical cells being rarely present;-asexual reproduction by zoospores; sexual reproduction mostly isogamous, rarely anisogamous; usually alternation of isomorphic generations.
Filamentous branched plant body; growth intercalary or trichothallic or apical without longitudinal subdivision of the cells of the filaments; gametangia or sporangia borne on all or part of an ultimate branch; gametes isogamous or anisogamous.
II. Order Laminariales:
Sporophytic generation represented by bulky parenchymatous forms of large dimensions with considerable morphological and anatomical differentiation having intercalary growth; gametophytic generation small and filamentous; asexual reproduction by zoospores formed in unilocular sporangia; sexual reproduction oogamous; heteromorphic alternation of generations.
Sporophyte of large and coarse fronds composed of holdfast, stipe and lamina; lamina is renewed annually in many species by a meristematic tissue present at the junction point between the stipe and the lamina; internal differentiation into medullary tissue, cortical layer and assimilating layer; lamina with sori bearing unilocular sporangia with paraphyses; zoospores borne in sporangia germinating to form gametophytes; oogamous sexual reproduction, fertilized egg giving rise to sporophyte.
III. Order Fucales:
Very large diploid plant body of complex morphological and anatomical differentiation, true parenchymatous tissue being formed with apical growth; asexual reproduction absent; oogamous sexual reproduction with sex organs borne in conceptacles in the plant body, meiosis during the development of gametes; haploid phase very short being confined to the gametes only; oospore directly germinates into a diploid thallus; no evident alternation of generations.
Perennial monoecious or dioecious plant body having holdfast and flattened dichotomously branched fronds, provided with air vesicles; conceptacles terminal or sub-terminal; conceptacles opening outwards by a narrow ostiole; antheridia on branching filaments; eight oospheres; fertilization outside the conceptacles.
Complex bilaterally symmetrical branched plant body; branches borne at the axil of leaf-like expansions; air bladders along the leaves or branches; monoecious or dioecious; oogonia liberated from the conceptacles are held by gelatinous stalks but fertilization outside the plant body; some species with cryptoblasts.
IV. Order Dictyotales:
Little differentiated expanded parenchymatous thallus with apical growth; asexual reproduction by non-motile tetraspores borne in a unilocular tetrasporangium; sexual reproduction oogamous; isomorphic alternation of generations.
Leafy olive-green thallus; asexual reproduction by tetraspores borne in tetrasporangia developed on the sporophyte; sexual reproduction by antherozoids and ova developed in sex organs on the surface of the gametophyte which may be monoecious or dioecious; sex organs in sori.
V. Order Desmarestiaies:
Heteromorphic alternation of a large erect, essentially pinnately branched sporophyte with a microscopic gametophyte; growth tri- chothallic initiated by a single filament; plants may be as large as many species of Laminaria, their structure simple in comparison with the Laminariales; the sporophytes bear unilocular sporangia; gametophyte plants, dioecious, and sexual reproduction, oogamous.
VI. Order Sphacelariales:
Present in all seas, and a species of Sphacelaria, the largest genus, may be expected in almost any intertidal flora; usually short, densely branched, tufted plants up to a few centimetres in height; possess well-marked apical growth from a large apical cell, parenchymatous structure, and usually radial organization.
VII. Order Tilopteridales:
Live mostly in deep water and sporadic and ephemeral; plants generally resemble Ectocarpus in filamentous habit, and show marked trichothallic growth; lower parts may, however, become multiseriate somewhat as in Sphacelaria; the characteristic reproductive structures globose or oblong monosporangia whose entire contents are liberated as a monospore; monospores quadrinucleate, and some cytological investigations suggest that the four nuclei are the products of meiosis; monospores, thus, may be considered a kind of primitive, undivided tetrad of spores; sexual generation produces plurilocular gametangia, but fertilization and other details have not been described.
VIII. Order Cutlerialess:
Both of the alternating generations are macroscopic but not always isomorphic; an advance toward strictly heteromorphic alternation of macroscopic and microscopic generations prevalent among the more highly developed brown algae.
IX. Order Sporochnales:
Mainly inhabitants of the Southern Hemisphere, but few interesting species of very wide distribution, occur along the North Pacific and North Atlantic; almost exclusively deep-water plants; all show an alternation of macroscopic sporophyte and microscopic, filamentous gametophyte; most distinctly characterized by the peculiar anatomy of the growing apices; the meristem at the branch tip sporophyte, consisting of a dome-shaped single layer of meristematic cells crowned by a tuft of simple hairs; detailed ontogeny of this mode of growth has been worked out only in the European genus Nereia.
X. Order Dictyosiphonales:
Structure of the thallus is parenchyma-like throughout and in which both longitudinal and transverse divisions of intercalary cells occur; characters of the subclass Polystichineae used by various authors to include the orders Dictyosiphonales (sometimes treated as Punctariales) and the Laminariales; in majority of Dictyosiphonales an alternation of a macroscopic sporophyte and a microscopic filamentous gametophyte (although there are several exceptions), but are, however, set off from the other orders by various characteristics their solid, parenchymatous structure easily sets them apart from the Chordariales; their lack of trichothallic growth, from the Desmarestiaies; their diffuse or apical growth, usually without a restricted intercalary meristem, from the Laminariales.
XI. Order Chordariales:
Trichothallic growth of the monoaxial or multiaxial sporophyte and isogamous gametic union (although gametes are still unknown for various genera and for most species); sporophyte plants range from minute, crustose epiphytes to erect, fleshy, or gelatinous plants up to 40 centimetres tall; gametophytes are microscopic, filamentous plants which have been observed only by culturing zoospores.