Perennation includes methods employed by plant to pass through unfavourable period for vegetative growth. In perennation period normally all the activities of organism cases and to pass through these unfavourable periods the living organism develops some stage in life cycle which is protected by thick persistent wall. Marine algae live in conditions where variations in external conditions of life are rare. The freshwater and sub aerial algae are however exposed to the dangers of desiccation and extremes of temperature. They develop certain devices which enable them to carry through the period of sever conditions into the next growing season. Protective devices constitute the means of permeation. Main methods of perennation is fresh water and sub aerial algae are (a) Akinetes (2) Aplanospores (3) Palmella Stage (4) Cysts (5) Bulbils and amylum stars (6) Oospore or Zygote.
(1) Akinetes:- These are formed during conditions of draught eg. In ulothric, cladophora, pithophora, oosgonium and nostor. Akinetes are produced ogengly in each cell by rounding off cytoplasmic contents of the cell. The thick wall of the spore fuses with the parent cell. Entherie filament dies leaving behind the akines. Thuge survive and develop into new plants with the return of normal conditions of life.
(2) Aplanospores:- In some algae like chlamydononas, vlotherix, coleochacte, oedogomium the plant cells form thick walled non motile aplanospores during unfavourable conditions. During their formation the cytoplasmic contents round off and develop thick walls. These are then liberated by dissociation of parent wall. After a period of rest at the approach of favourable conditions they develop into new plants.
(3) Palmella Stage:- Under advese conditions in some green algae such as chlamy domoras, the vegetative cell divide, to form new daughter cell. These increase in number by repeated cell division. All the cells remain clustered together inside mucilage envelop formed by gelatinisation of parent cell wall of the successive generations. This is the palmella stage. It is of short duration and represents milder method of perennation.
(4) Cysts:- During unfavourable conditions the plant body of certain algae like vaucheria becomes separate. Each segment germinates directly into a new plant on the approach of favourable conditions.
(5) Bulbils and Amylum Stars:- In chara the tuberous bulbils and amylum stars also serve as a method of perennation. The amylum stars contain a large amount of starch gramules. They germinate.
(6) Oospore or Zygote:- Formation of zygotes method of perennation in green algaye. The zygote which is formed as a result of sexual fusion are thick walled structure with plenty of food material. Zygote usually undergoes a period of rest tot tied over the unfavourable conditions and then germinate into new plants either directly or indirectly by forming zoospore.
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