Companion cells in the secondary phloem of Indian dicotyledonous species: a quantitative study

Abstract

Companion cells in the secondary phloem of 125 Indian dicotyledonous species belonging to 43 families were examined by light microscopy. Four types of companion cell were identified: E-, S- and L-types which were equal in length, shorter, or longer, respectively, than the associated sieve tube element; and R-type, in which two or more companion cells in a vertical row were associated with the sieve tube element. The commonest was the E-type and the rarest was the L-type. E-type companion cells were most frequently found associated with short sieve tube elements (50–250 μm) which had a high frequency of simple sieve plates, considered phylogenetically advanced. R-type companion cells were most frequently associated with long sieve tube elements (>400 μm) with a high frequency of compound sieve plates, considered phylogenetically the least advanced. A strong positive correlation was found between the average number of companion cells associated with a sieve tube element and the lengths of the sieve tube elements. There was also a strong negative correlation between the average number of companion cells associated with a sieve tube element and companion cell lengths.