Although there are freshwater species the majority of algae live in the sea and the various seaweeds found at low tide are familiar examples. A lot of the most primitive of these plants are single-celled and invisible to the naked eye, whereas the large, more advanced seaweeds are made up of countless cells which may be divided into groups or tissues performing different tasks. Such plants may have primitive roots. These algae are very similar to other plants in the way they obtain their food, for they contain chlorophyll and photosynthesis takes place in the same way as it does in the advanced forms of plant life. Although many are green, quite a number have pigments in their cells which are stronger colours than green, giving the algae red, brown or even bluish colours. Their method of increase can be either by simple division of the original plant or sexual reproduction. There are thousands of different species of algae and it would be impossible here to describe even a small proportion of them, but one or two examples can be taken to show what life-forms exist in this group. Some of the tiny single-celled species are capable of movement and such a plant is Chlamydomonas, which is found in ponds and ditches. It is roundish in shape and is built up in the same basic way as other cells. At one end are two thread-like arms which project through the cell wall. These are called flagella, and by waving them around the cell can move in the water. The cell contains a nucleus and chloroplasts for manufacturing food, and in addition an orange-coloured object known as the eye-spot. This helps the chlamydomonas find its way towards brighter light, and in doing so gives the chloroplast a better chance to produce food. Other single celled algae do not go through life individually, but group themselves into a calony and hundreds or thousands together may just be visible without a microscope. These types are interesting in that they all use their flagella together and move as a whole colony towards light. Sometimes in bright weather the rate of reproduction is so fast that the water in ponds may be coloured green by their presence, and can easily be seen. The hard green covering often seen on older wooden fences is formed by large numbers of a small land alga known as Pleurococcus. It can withstand long periods of drought but like those which live in water it only becomes very active when there is plenty of moisture and it is warm. Many of the other algae in water can also form what are called resting stages so they can withstand adverse conditions such as the drying out of the pond in which they live. One of the commonest alga in ponds and slow steams is the plant which sometimes forms the slimy green masses familiar to all who spend some time near the water. This is called Spirogyra and is a long thread like plant formed from short cylindrical cells joined end to end. Among the most advanced of the algae are the common forms of seaweed called wracks which inhabit rocky costs all over Europe. Fucus vesiculosus commonly called bladder wrack, has a portion as its base which is adapted to clasp on to rocks and prevent the plant being washed away. This part of the plant has no powers of absorbing foods and is entirely an anchor. The stem is cylindrical at first then higher up it is flattened out and has built in bladders of air which act like water wings and keep the plant upright in the water. The whole plant is very slippery, being covered with a jelly like substance and it is rubbery to withstand the buffeting of waves. Reproduction is either by the simple breaking off of sections of the stem which in these plants is called the thallus, or by the formation of two kinds of special cells, one of which is capable of movement like chlamydomonas plants.These special cells are set free when the tide is in and the mobile ones swim to the others and they fuse together forming a spore. This can germinate and develop into another plant.