Sometimes, as a result of accident or by design, an organism loses a part of itself, whereupon it attempts to repair the damage. The extent to which organisms can repair themselves varies greatly. Plants can survive extensive damage ; as a rule, the loss of parts is followed by rapid growth or development of similar parts elsewhere on the plant. This is well exemplified by theof fruit trees and the disbudding of . Among animals, however, the ability to replace lost parts is confined to the lower animals. Hydra, if cut into two horizontally, can grow a new set of tentacles on the old body whilst the other half grows a new foot. Until the Abbe Trembley discovered this fact in the middle of the eighteenth century, the ability to replace lost parts was considered to be a characteristic feature of plants. So many cases of regeneration are now known among animals that we can no longer hold this view. The free-living flatworms are remarkable, for they can be chopped into pieces and each piece will grow into a complete flatworm, which is minute since it cannot during the process and has therefore to build up missing parts from the material of those already existing. The earthworm cut in two above the clitellum will gradually develop into two complete individuals. A single arm of a starfish can grow four small new ones. A lobster as a result of fighting may lose a claw, or even a whole limb. When it moults, the missing part is reformed in miniature, becoming bigger with each moult.
The best example among vertebrates is the newt, which can regenerate a limb or the tail which has been bitten off. The new structures are quite normal and differ in no way from the ones which have been lost. Lizards which hide in crevices in the rocks in sandy places often shed the tail when it is grasped by an enemy. In this case, although a new tail is grown, its skeleton lacks the typical structure of vertebras.
In higher animals, such as man, regeneration is limited to tissue repair, whereby wounds are healed. A structure once lost is not replaced, unfortunately. We can therefore trace a gradual loss of ability to regenerate on going from simple to complex animal forms, complete regeneration being shown by Hydra and flatworms, body regeneration by earthworms, organ regeneration by Crustacea, Amphibia and lizards, and tissue regeneration only in birds and mammals. This loss may be traced to the specialization of cells, for cells in higher forms only carry out particular functions, the highest degree of development in this way being apparently the nerve cells in the brain of man which, once formed, are never replaced. Other types of cells when damaged can be destroyed by white blood corpuscles, and their place taken by new ones formed by cell division ; but the only repair possible with nerve cells is the replacement of parts of cells, and this only occurs when the undamaged part contains the nucleus. Our length of life would then seem to be the length of life of our nerve cells, unless we can discover some means of regenerating complete nerve cells.
There are occasions when at least one factor, e.g. temperature, light, water supply, is unfavourable to a particular organism. The way in which the organism is affected by and responds to each factor varies greatly, but the simpler forms quite commonly live through an unfavourable period in an ’encysted ’condition. Among the Protista, the protoplasm usually forms into a spherical mass ; a thick wall is formed around it, the whole structure then being called a cyst. In this condition the organism can withstand wide variations of temperature and resist desiccation. All normal activity is apparently suspended and, in particular,and growth.
Many higher forms survive the winter by hibernation, e.g. hedgehog, migration to warmer places, e.g. birds, or by being adapted so as to be somewhat independent of the environment, e.g. against cold by hair or feathers or by thick deposits of fat beneath the skin, but above all by their circulating warm blood.
DISCONTINUOUS GROWTH: REPRODUCTION
We have already stated that the organism is fully developed when it can reproduce, I.e. bits of itself can develop into new individuals. These ’bits, ’ reproductive bodies, are of two kinds : a single unit of protoplasm which separates from the adult and grows ultimately into a similar adult—this we call a spore, or daughter cell ; tzvo units of protoplasm, generally from two different adults, come together and mingle to form a single mass which grows into an adult. The two units are termed gametes, and the single mass a zygote. The production of new individuals is sometimes called discontinuous growth, since it is not one and the same individual which is growing all the time.
The fusing together of two similar gametes is called conjugation. In higher forms of life the two gametes which fuse are dissimilar and are produced by organisms of different sexes, and since this is the only mode of reproduction in the majority of cases, we call it sexual repro- duction. Asexual reproduction, therefore, is the production of new individuals by a process not involving fusion, e.g. by spores. There are two ways in which organisms produce spores or gametes : by fission, when the whole organism splits up to form them, e.g. in Amoeba, and when only a part of it is concerned with producing them, the rest of the organism being termed its body or soma. In animals possessing a soma the gametes are produced in organs called gonads. The male gonad producing sperm is called the testis, while the female gonad producing eggs is called, the ovary. The first instance of this we met was in the study of Hydra.
Types and Examples of Reproduction. A. Asexual Reproduction—
Binary fission, e.g. Amoeba, Protococcus, Paramecium, Spirogyra, bacteria.
Spore formation, e.g. Mucor, mushroom, fern.
B. Vegetative Reproduction
E.g. Hydra, yeast,.
C. Sexual Reproduction—
By conjugation of isogametes, e.g. Spirogyra,
By fusion of heterogametes, e.g. Paramecium, Hydra, earthworm, insect, fish, frog, bird, mammal, Fucus, fern, flowering plant.
The processes giving rise to spores and zygotes are processes of reproduction differing from the other modes in that the organism develops from a single cell with nucleus and cytoplasm comparable to the simplest unicellular organisms. Since this single-celled stage by continued fission, acquiring nourishment and growing, can develop into a highly complex form of life like a flowering plant or a vertebrate animal, it must possess oil the potentialities of the complex adult form, for it normally becomes like the organism which produced it.
The ferns—together with mosses, liverworts, horsetails and many coelenterates—exhibit also an alternation of generations.
The flowering plants also exhibit alternation of generations, although in this case it is not so apparent:—