ANIMALS AND PLANTS: THEIR SIMILARITIES AND DIFFERENCES

SIMILARITIES BETWEEN ANIMALS AND PLANTS

All organisms are built up of the same units, each unit, the cell, consisting of a nucleus embedded in cytoplasm which is enclosed by a membrane. /// all organisms new cells arise by the division of existing cells by biliary fission, followed by the growth of the daughter cells. In the Protista the daughter cells separate to form distinct individuals, but in all other cases a multicellular organism results. Such cell division is best seen at the growing point of root and shoot in a plant and in the embryos of animals.

All protoplasm is unstable, I.e. it is constantly breaking down, and therefore must be renewed ; for this purpose materials and energy are required—both of zvhich all organisms obtain from food. This is the primary purpose of food for all organisms.

All organisms also require food for making additional protoplasm, or for supplying material to repair parts which have become accidentally damaged, for supplying material with which to build up the non-living parts such as those of the skeleton, e.g. calcium phosphate and chitin of animals, the celluloc.; and lignin of plants, and for supplying energy for all the other activities.

For purposes of transport all organisms convert their useful food into a fluid condition by digestion, e.g. in the alimentary canal of animals and in storage organs in plants. In higher forms food is transported in vessels, e.g. in blood vessels or along the sieve tubes of the phloem.

They store foods which are chemically similar, e.g. carbohydrate and oils in plants, carbohydrate, oils and fats in animals.

The higher forms get rid of water by pores in their surface : the plants using stomata and lenticels, and animals using pores in the skin.

The majority of animals and plants reproduce by conjugation of gametes—this mode of reproduction being the only one exhibited in higher forms of life.

With few exceptions, the male gametes of animals and those of plants which live in a watery habitat are alike in possessing cilia or flagclla, enabling them to swim to their female gametes.

All exhibit some type of movement.

All are irritable to external stimuli.

DIFFERENCES BETWEEN ANIMALS AND PLANTS

We can trace the differences between animals and plants to a difference in their modes of nutrition. To make the contrasts more obvious they are summarized on pp. 415 and 416.

An examination of these will show that there are many exceptions in minor respects. Bacteria have no well-defined nucleus, but possess nucleoplasm which is spread throughout the cytoplasm. Many unicellular organisms have some of the characteristics of both animals and plants, and are therefore claimed by both botanists and zoologists in their classifications. Some biologists place these unicellular organisms in a group distinct from both, terming them Protista. A typical example is Euglena.

The fungi lack chlorophyll and can build up proteins from amino-acids and peptones : otherwise they are typically plant-like.

The insectivorous plants, e.g. sundew, cannot obtain nitrates through their roots since they live in places where the soil lacks them. They therefore trap insects and pour digestive juices containing enzymes over them, so that proteins are thereby dissolved and the soluble products, probably amino-acids, are absorbed.

The tapeworm is diffuse and has no alimentary canal, but it lies in the alimentary canal of its host, bathed in digested food, so that its surface provides a sufficiently large area through which to absorb all the food it needs.

Spiders suck up the juices of their prey; common flies eject fluid over sugary food and suck up the resulting solution ; plant lice live upon the fluid from soft plant tissues ; whilst all the mosquitoes live upon fluid food, the females, like fleas, piercing the skin of animals to suck their blood.

In other respects all these organisms—excepting the Protista— quite obviously belong to the particular organic kingdom in which they are placed, their peculiarities serving to show how all environments and possible sources of food are explored by them.

ANIMALS

Animals consume ready-made food. This, being vegetable or flesh food, is organic in origin, and animals are therefore dependent upon the activities of other organisms for their food supply, and they have to move to get food, and are therefore compact.

They grow only until adult and their growth is intercalary ; the shape and number of their parts are fixed.

Their food is generally solid.

They can live in the dark for long periods.

They do not possess chlorophyll.

Food is ready-made and, being solid, must be digested prior to transport to all the cells of the animal.

They are generally very active and their locomotion requires the expenditure of much energy. Therefore the rate of living is high, and consequently much waste is produced as a result of tissue respiration, and since much food is required for this purpose they are essentially katabolic.

They are sensitive to many stimuli. The possession of sense organs enables them to become well acquainted with their environment, whilst the nervous system enables them to make rapid responses which are infinitely variable, reversible and temporary.

The cell is usually bounded by a cell membrane ; a wall, if present, is never thick and is never made of cellulose. Vacuoles are always numerous and very small. Chloroplasts or leucoplasts are never found in animal cells. There is a wide range of difference between one type of cell and another, e.g. muscle cells, neurons, adipose tissue.

GREEN PLANTS

Plants are surrounded by their material requirements. These are inorganic substances rendering the plants independent of the activities of other organisms. They have no need to move to obtain these substances.

The raw materials are absorbed in fluid form, and in order to absorb them more readily plants are diffuse with large surfaces for absorption. They exhibit unlimited growth, mainly apical, and the shape and number of their parts are continually changing. From the raw materials food is built up by photosynthesis, light being essential for this process and also chlorophyll. Food is produced in soluble and diffusible form, so that digestion is unnecessary. There is little movement by plants. Therefore little energy is expended in this way.

Therefore : The rate of living is low, and little zoaste is produced as a result of tissue respiration.

Also : This fact, together with the fact that green plants make their own food by photosynthesis, shows that they are essentially anabolic.

The chief stimuli producing responses are three : light, water and the force due to gravity. The responses are usually tropisms which are made slowly, are limited in nature and usually irreversible and permanent, e.g. growth response to gravity.

The cell unit is nearly always bounded by a wall which is thick and is made of cellulose.

There are typically few vacuoles which are relatively large. Chloroplasts or leucoplasts are present in cells where starch occurs. Although varying in size, all the cells are roughly prisms in shape.

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