<Li> Midvein (Primary vein) </Li> <Li> Secondary vein . </Li> <P> Although leaves can be seen in many different shapes, sizes and textures, typically a leaf is a thin, dorsiventrally flattened organ, borne above ground and specialized for photosynthesis . In most leaves, the primary photosynthetic tissue, the palisade mesophyll, is located on the upper side of the blade or lamina of the leaf but in some species, including the mature foliage of Eucalyptus, palisade mesophyll is present on both sides and the leaves are said to be isobilateral . Most leaves have distinctive upper surface (adaxial) and lower surface (abaxial) that differ in colour, hairiness, the number of stomata (pores that intake and output gases), epicuticular wax amount and structure and other features . </P> <P> Broad, flat leaves with complex venation are known as megaphylls and the species that bear them, the majority, as broad - leaved or megaphyllous plants . In others, such as the clubmosses, with different evolutionary origins, the leaves are simple, with only a single vein and are known as microphylls . </P>

Which tissue is present in the veins of leaves