Variation in Plants (With Diagram)

The below mentioned article provides an overview on the variation in plants.

With the growth of this science, Bateson coined the word Genetics in 1906 for that branch of Biology which is concerned with the “elucidation of the phenomena of heredity and variation”. The word itself is of Latin origin and means the genesis or origination of organisms.

Genetics seeks to account for the resemblances and differences which are found among organisms related by descent. Heredity is the transmission of parental qualities, expressed or latent, to the progeny. Thus Genetics is the critical study of heredity enabling one to understand the mechanism of Evolution. Eugenics is the science of Genetics applied to human beings with special reference to race betterment.

The science of Genetics began with a study of variations to find out which of them were inherited.

Variation:

The actual build up of any organism is determined by two factors—heredity and environment, i.e., nature and nurture. We have seen that while Lamarck stressed the importance of environment (in addition to the ‘conscious efforts’ of animals) alone, Weismann went to the opposite extreme and, at first, completely ignored the effect of environment.

But it is clear that if heredity be so constant and immutable then new species cannot be formed and no evolution can take place. To explain evolution there must be variation. Variation is of the greatest importance in evolution. Nature selects out those hereditary variations which render the organism more suitable to its environ­ment.

Thus, the selected variants become better adapted to their surroundings and, in course of time, progressive selection forms a new species. So, variation, selection and adap­tation principally determine the course of evolution. Adaptations are special modifica­tions which suit the particular environment of the organism.

The most bitter arguments in Genetics and Evolution center around the problem of the origin of variations. This is a matter of contention between the neo-Lamarckians and the Weismannians or neo- Darwinians. Some vitaligt philosophers (Bergson, Smuts, Bernard Shaw, etc.) talked of some supernatural force or some Life Force causing evolution in certain directions. But these are merely speculative and not based on actual facts. At present, all arguments have condensed down to two main channels—either heritable variation are caused by accidental changes in the germplasm or, environment moulds such variations.

Most geneticists today believe in accidental evolution caused by the accidental variations or mutations. It is possible that some of the mutations are due to disturbances caused by environmental changes. On the other hand, there are others who believe that new true breeding variations arise as a result of the direct effect of the environment. The Russian Lysenko school of Genetics, who claimed that they alone were the true followers of Darwin, were strong advocates of the latter view.

Certain phenomena in evolution have been differently explained by the vitalists, the mutationists and the environmentalists. Orthogenesis means evolution in a definite direction. That is how the seed habit in plants and the brain in animals developed. The vitalists argue that a directive force caused this evolution and ‘predetermined’ the direction of orthogenesis.

The mutationists, on the other hand, say that nature selected out such mutations while the environmentalists would say that environment moulded out these forms. Epharmosis means that evolution of similar forms in different groups by the same environment. Thus, cushion and rosette forms have been developed in alpine plants of different types. Xerophytic cacti and xerophytic Euphorbias look much alike though they come from Widely different groups.

This phenomenon is also known as homoplasy. This convergence in evolution caused by epharmonic adaptation has given rise to much speculation. Parallel evolution of heterospory in different Pteriodophytic groups may also be ascribed to similar reasons. Vavilov formulated an interesting Law of Homologous Series in Variation by showing that the different cultivated crops follow the same course in the evolution of new varieties. This also provokes arguments as to whether such parallel evolution can be accidental.

Organisms with the same heredity have been found to show the following types of variation:

(1) The fluctuating or continuous variations are very common. It is very difficult to find two leaves on the same plant which are exactly alike. This variation is caused more by ‘nurture’ than by hereditary ‘nature’. Internal and external environment causes such variations and these are not inherited.

The original concept of fluctuating variations, as understood by Darwin and others, however, included something more than what is stated above. This includes the segre­gates of polygenes, i.e., the multiple genes and such minor polygenes and modifying genes which are constantly arising in nature by mutation.

(2) Mutations or discontinuous variations appear suddenly. They are caused acci­dentally, or as an effect of environmental changes.

(3) Variation by hybridisation and recombination of the germinal material. Mendel is called the Father of Genetics as he was the first scientist who demonstrated how hybridisation could be utilised in understanding the heredity of organisms.

(4) Chimaeras. In grafts between two plants sometimes some peculiar buds occur at the point of union of the stock and the scion (Fig. 814A). The tissue of such a bud is actually a mixture of the tissue of the two plants.

Branches arising from such buds and plants vegetatively propagated from them show some mixtures of the two parent plants simulating true hybrids and are known as chimaeras. Chimaeras are also known to arise on normal plants without grafting, being caused by mutation of somatic cells, i.e., bud mutations.

Chromosomal mutations are known to take place in the somatic tissues of plants so that two tissues, with cells which are genetically different (i.e., with difference in chromosomes), come to live side by side. This is called a chromosomal chimaera. A bud developing out of such a tissue will be a chimaeral bud mutation. A very interesting chimaera is the gynandromorph form of insects, half of which is male and half female as one-half of the insect is developed of male cells, and the other half out of female.

There may also be a chimaera involving more than two types of gene­tically different tissues. This is termed a policlinal chimaera.

The two types of parental tissues in a chimaera may be arranged differently and different types of chimaeras are recognised according to this. Fig. 814B shows the arrangements in (a) periclinal (one tissue on periphery only), (b) sectorial (two sections occupied by two tissues) and (c) hyperchimaeral (two tissues mixed up indiscriminately) types.

The term graft hybrid has been used for similar variations by some classical authors, but, in a different sense. The ‘graft hybird’ hypothesis supposes that there is actual hybridisation or, rather, nuclear fusion between the cells of the stock and the scion. Tissues may separate out later by the segregation of these fused cells. The same term ‘graft hybrid’ had, again, been used in a different sense by the Russian Lysenko school.

Winkler (1911) obtained remarkable chimaeras from Solanum nigrum (woody night­shade) x Solanum lycopersicum (tomato) grafts (Fig. 815) all of which could pass as true hybrids. But, on closer examination, these ^‘graft hybrids’ were found to be periclinal chimaeras. Cytisus adami, a periclinal chimaera between Laburnum vulgare and Cytisus purpureus, is also famous.