In this article we will discuss about the distribution of stomata on leaves.
Stomata most frequently occur on regions over the chlorenchymatous mesophyll cells of leaf. Stomata do not occur over the fibre bundles of veins. They also do not occur over the sclerenchyma present on the leaf margins.
In majority of monocotyledons where the veins are parallel, e.g. Graminsae, the stomata are arranged in rows parallel to the long axis of leaf. In dicotyledons where the venation is reticulate there is no particular order of arrangement of stomata. Here the stomata are irregularly distributed over the leaf surface, but they are roughly equidistant apart from each other.
This is due to the fact that the stomata are mutually inhibitory, that is, existing stoma inhibits the formation of new one. It is regarded that there occurs an inhibitory zone around a developing stoma that prevents to develop another stoma. It is suggested that there may be hormonal control behind this phenomenon.
It is observed that in mustard cotyledon stomata do not fully develop in dark. But when the mustard cotyledons are exposed to far-red illumination fully developed stomata result. So it is thought that the hormone phytochrome is involved in the development of stomata.
There are reports of unusual distribution stomata over leaf surface. In Begonia semperflorens stomata occur in clusters of two to four each. In- Nothofagus nitida the clusters of stomata are much larger than Begonia. In Saxifraga stolonifera (formerly sarmentosa) stomata are located on raised patches. Unusual pattern of distribution is also observed in the different species of Saxifraga.
In certain species stomata occur near leaf margin while in other species stomata only occur near leaf tip. In Daphne petraea stomata occur only in two bands immediately adjacent to the midrib. Mimosa cruenta has stomata distributed throughout the upper surface. On the lower surface stomata are restricted to just one of the two longitudinal halves of the leaflet.