Science divides all the diversity of the surrounding plant world into two sub kingdoms: higher and lower plants. Higher plants include plants whose vegetative body is divided into organs; they have a stem, roots and leaves. In lower plants, on the contrary, there is no such separation, in addition, there are no multicellular reproductive organs. In modern biology, only algae are referred to lower plants, and a few dozen years ago fungi and bacteria, as well as all organisms, except animals and higher plants, were ranked among them.
The lower plants are characterized by: diverse color, unicellular reproductive organs, aquatic habitat. Algae on our planet appeared first: the age of fossil plants found in the layers of the Archean and Proterozoic era is about 3 billion years.
Some, focusing on the name, think that lower plants are primitive unicellular microscopic size. However, this is not quite true. Of course, unicellular algae belong to this sub kingdom, but it also includes large, multicellular algae, whose length reaches several tens of meters. Like the higher ones, they can participate in the process of photosynthesis. Propagation of algae occurs sexually and asexually (vegetatively or by zoospores). All lower and higher plants contain chlorophyll, but lower ones have, in addition to this pigment, others that give them a specific color: brown, yellowish, red, etc.
Lower plants, examples
Algae can be divided into marine and freshwater (most of them). Lower plants living in sea water can be located both on the surface and in depth. However, for life they need light, therefore, at great depths - of the order of 250-300 meters or more - they can not be found, because the rays of the sun do not penetrate the water column.
The most famous seaweed is kelp or seaweed . It is a narrow leaf, reaching a length of several meters and clinging to the bottom of the processes of rhizoids. Laminaria propagates by zoospores - cells resembling a pear in shape, containing a core and chromatophores, equipped with flagella for movement. After leaving the mother cell, the zoospore picks with flagella, moving in water until it attaches to a nutrient substrate and gives life to a new alga.
Fucus is another species of marine brown algae. This multicellular plant looks like a bush; its length can reach two meters. Fucus is attached to the nutrient substrate with the help of the sole, often forming whole thickets in coastal waters. It reproduces sexually: in some maternal cells, called anteridia and located along the edges of the plant, spermatozoa are formed, in other cells - oogonia - eggs are formed. Under the influence of attractants - substances that attract sperm in the water to the eggs, they merge and a zygote forms, from which new algae subsequently grows.
In fresh water bodies, lower plants (algae) are also found. The most common of them is spirogyra. Spirogyra is easiest to meet in bodies of water with stagnant water: bright green mud forms clusters resembling slimy cotton wool. If you look at it under a microscope, you can see the tall plant, consisting of large (up to 0.01 mm long) cylindrical cells elongated in a row. Spirogyra can reproduce in two ways: sexual and vegetative. Unlike fucus, in which the formation of a zygote occurs when the egg and sperm merge, in this alga, a zygote forms when two cells merge. Vegetative propagation occurs when the strand breaks, in this case a new plant is formed from each part.