Raphe - (1) An elongated mass of tissue having a vascular bundle, and lying on the side of an anatropous ovule, between the chalaza and the attachment to the placenta.
(2) A slit-like line which is running longitudinally on the value of a diatom to indicate the position of a narrow slit in the wall; it bears a nodule at each end and one in the middle.

Raphide - A needle-shaped crystal; generally of calcium oxalate, which is found in bundles, rounded masses, or singly in certain plant cells.

Raunkiaer System of Classification - A method of plant classifi;cation postulated by Danish botanist C. Raunkiaer. In this system plants are divided into groups depending upon the degree of the protection in condition of drought or/and cold and the position of perennating organs. System funds use in ecological grouping of vegetation in different regions and com paring these groups in different climatic conditions. The various categories of plants are chamaephyte, cryptophyte, hemicryptophyte, phanerophyte; therophyte, etc.

Ray - The non-vascular tissue which is developed in a stele. Be;tween the primary vascular bundles there is the interfascicular ray, and that developed in the secondary vascular tissue, by division of the cambium, there is a vascular ray.

Ray Floret - One of the small flowers radiating out form the margin of a capitulum or other dense inflorescence.

Ray Initial - One of the cell of the cambium that is taking part in the formation of a ray.

Ray Tracheid (e). A thick walled cell occurring in the vascular rays of pines. It possesses bordered pits and conducts aque;ous solution horizontally.

Reaction - Any change taking place in the activity of an organism in response to a stimulus.

Reaction Time - The time interval between the application of a stimulus and the appropriate reaction.

Reaction Wood - Used for structurally abnormal wood which is formed in response to various stresses. In gymnosperms, it is called compression wood and is formed on the lower sides of branches. It is dense due to heavy lignification. In angio;sperms, it is called tension wood which is formed on the upper side of branches and fibres are gelatinised rather than ligni;fied.

Recalcitrant Seed - A losing viability on storage for any length of time even under conditions normally used for seed storage e.g., seeds of many tropical plants e.g., coconut, rubber, tea, etc. Such seeds can be stored for only one season.

Recapitulation Theory - Theory postulated by Ernst Hackel. According to this ontogeny of orga1n or organism is a speeded up recapitulation of its phylogeny and the developmental stages resemble adult forms of ancestors in evolutionary sequence. Theory has been discarded by many.

Receptacle - (1) In fungi, a spore-bearing structure, especially if it is more or less concave.
(2) In algae, the swollen end of a branch which bears repro;ductive organs.
(3) In liverworts, a cup which has gemmae.
(4) In ferns, the cushion of tissue which bears the sporangia.
(5) In flowering plants, (a) The more or less enlarged end of the flower-stalk which bears the flower-parts; (b) The enlarged end of a peduncle which bears the flowers of a crowded inflorescence.
(6) In fungi, an axis which has one or more organs.

Receptive Body - A small branched or unbranched process from the stroma, which is capable of being 'spermatized' by micro;condia.

Receptive Spot - The clear area in the female gametes of some fungi and alage, through which the male gametes are enter;ing.

Receptiveness - The condition of a stigma when effective pollina;tion, and fertilization is possible.

Recessive - Used for the allele which can be expressed only in homozygous condition and its expression gets suppressed in heter-ozygous condition by the other (dominant) allele.


Reciprocal Cross - A cross in which the source of male and female gametes has been reversed. Such cross is mainly used for identifying influence of maternal (or paternal) factors, if any, on the inheritance of characteristics.

Reciprocal Translocation - Describing translocation which in;volves exchange of segments mutually between two non;homologous chromosomes.

Recombinant DNA - (1) The genetic material in which crossing over or chromosome reassortment has taken place naturally. (2) The genetic material having novel gene sequences which is produced by techniques of genetic engineering (recombinant DNA technology).

Recombination - The formation in the offspring of combinations of genes not present in either parent. It is carried out by crossing-over or segregation at meiosis, and/or the random union of different sorts of gametes at fertilization.

Recon - The unit of inheritance which gets transferred as a unit during recombination. This definition has been found to be true for genes of most of the characters but not in all the cases.

Recurrent - Used for the small veins of a leaf, when they bend back towards the mid-rib.

Recurrent Parent - Used for the parent of cultivated variety which has been backcrossed with the donor parent and the back;cross progenies repeatedly.