The various vegetative organizations of Phaeophyceae may be classified in the following groups.
No member is unicellular, colonial or unbranched filament.
The simplest member is heterotrichous filament, which shows the highest organization in Chlorophyceae is the lowest organization in Phaeophyceae.
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Ectocarpoid:
In Ectocarpus the thallus is profusely branched and the cells are joined end to end in a single series. The thallus consists of a prostrate branched portion, from which the erect branched system arises. This is heterotrichous type of organization.
Cable type:
This type is represented by Mesogloea. Here the thallus consists of several axial strands like a bundle. Some radial branches come out at right angles from these threads, and the whole structure appears like a cable.
Multiseptate cable type:
This type is represented by Chorda filum. The thallus is formed by many compact filaments. Actually there is no parenchyma but pseudo parenchyma.
Corticated type:
Desmarestia, Arthrocladia, etc., represent this type. Here in the lower region of the plant body the lateral branches become rhizoidal and coil around the main axis to form a compact pseudoparenchymatous cortex around the main axis.
Truly parenchymatous forms:
In such forms the thalli are leaf-like or laminate and truly parenchymatous, e.g., Punctaria. The members of Laminariales and Fucales possess improved type of parenchyma in them. The plant body of Macrocystis is 30 to 50 metres long. It has a dichotomously branched holdfast, a stipe and large leaf blades having gas bladders at their bases. This is the longest known giant kelp.
The plant body of Nereocystis is 20 to 25 metres long. It consists of a haptera, a stipe and terminal large gas bladder having many blades on it.
Laminaria, Fucus, Sargassum, Postelsia, etc., all possess truly parenchymatous structures.
Internal structure:
In Laminaria, the stipe consists of epidermis, cortex and medulla. There are ‘trumpet hyphae’. At the junction of the mouths of two trumpets a callus pad develops in winter and dissolves in spring. This refreshes the memory of the callus pads of the sieve tubes of higher angiosperms.
Hairs:
The hairs are present in all members of Phaophyceae. They are multicellular and generally ensheathed at their bases.
Cell structure:
The cell wall is two layered. The inner layer outer to the protoplast consists of cellulose and the outer layer of mucilaginous substance. The mucilage contains gum like substances such as algin, fucoidin, etc. Callose is found in the cell membranes of Laminaria digitata. The callus pads consists of callose. In Laminariales special mucilage ducts are found, secreting mucilage.
The protoplast:
It is less viscous. The cytoplasm contains mitochondria which are in contact of chromatophores. The cell contains numerous vacuoles. In Dictyota and Fucus each cell contains a large vacuole. In cell sap, the colloidal substances are found.
The special bodies called fucosan vesicles are abundantly found in meristematic, photosynthetic reproductive cells. They are highly refractive, colourless and rounded bodies.
Generally the chromatophores are discoid and parietal in position. In each cell, there may be one or more chloroplasts. The pyrenoids are usually attached to the inner margins of the chloroplasts. In many cases the pyrenoids are stalked and attached laterally to the chloroplasts.
The chromatophores contain chlorophyll a, chlorophyll b, xanthophyll and carotenoids as pigments. Fuco or phycoxanthin overmasks the other pigments, giving a tinge of brown colour to the plant body.
Photosynthetic products:
According to Kylin, simple reducing sugar such as dextrose or pentose constitute the first products of photosynthesis. He also reports a disaccharide known as laminarose as first product of photosynthesis.
Food products:
The important food products are (1) laminarin (2) certain hydrolases (3) mannitol and anhydride manniton (4) fats and (5) certain oxalates. Besides, in Laminariales and Fucales considerable quantity of vitamin C is found. Iodine is stored in many algae. In Laminaria (kelps) the percentage of iodine ranges from. 08 to 3.5.
Nucleus:
Majority of Phaeophyceae have uninucleate cells. Multinucleate condition is rare. The nucleus is quite large in size. The chromatin material is found in less quantity. The nucleus contains one or two nucleoli. In Fucales the nucleoli are vacuolate. The extra or intranuclear centrosome is also present. In Phaeophyceae the number of chromosomes is from 8-32. The basic number is 8.
Swarmers:
The swarmers may be zoospores or gametes. They are ovoid or pyriform. The swarmers are biflagellate and flagella are unequal in size. In majority of Phaeophyceae the flagellum directed forward is larger and directed backward is smaller in size. In order Fucales the position of flagella on the swarmer is reverse.
Here the flagellum directed forward is smaller and directed backward is larger in size. In order Dictyotales the swarmers are single flagellate. The flagella are attached laterally to the swarmer. Each swarmer contains a large nucleus, 3 or 4 chromatophores, a stigma, a blepharoplast and a plastomere.