Semantides (or semantophoretic molecules) are biological macromolecules that carry phylogenetic information about evolutionary history. Semantides may be further classified into three categories: the genes themselves are known as 'primary semantides' , the mRNA transcripts 'secondary semantides', and polypeptides 'tertiary semantides'.

The terms were coined by Linus Pauling and Emile Zuckerkandl and used to contrast to episemantic and aposemantic molecules when discussing the phylogenetic evolutionary history contained in macromolecular sequences. Although semantides are the major type of data used in modern phylogenetics the term itself is not commonly used.

The macromolecules chosen are usually common to all cells, used in phylogeny because they change slowly over time. Examples of these molecules are:

• rRNA/rDNA
• RNase P RNA
• ATPase
• RecA protein (involved in genetic recombination)
• Cytochrome c
• Heat shock protein genes

The above semantides can yield different phylogenetic trees, but if the genetic relatedness is correct for 2 or more, that correlation could mean that the evolutionary tree is correct.

Among the various semantides, proteins serve as the most important tool in chemotaxonomy.

The importance of proteins in chemotaxonomy has several reasons behind it:

I. Firstly, these are large, complex molecules showing little qualitative variation with changing environmental factors.

II. Secondly, they are universally distributed.

III. Thirdly, they are relatively simple to extract and handle and present in appreciable amount.

IV. Lastly, numerous cheap, simple and rapid methods have developed in the recent past, for protein analysis and comparison by taxonomists, of which electrophoresis is particularly an important method of protein separation.

The variation in protein structure between plants effectively provides an inner view into the cell’s genome, as the amino acid sequences of proteins are encoded by the nucleotide sequences of a cell’s DNA.

Proteins have been used in taxonomic investigations in the following ways:

Comparison of the protein banding patterns :

In recent times protein banding patterns obtained by gel electrophoresis, have been focused on the problem of identification of critical taxa, their relationship and taxonomic status. Taxonomic interpretations should be based on comparison of proteins from homologous organs of the same age in order to