Systematic anatomy of the primitive cereoid cactus Leptocereus quadricostatus. The genus Leptocereus is widely regarded as one of the most primitive of Cactaceae subfamily Cactoideae. It resembles the pereskias in having dense fibrous wood in its trunk and lower branches; this wood lacks vascular tracheids; it has septate protophloem fibres; and the ray cells are all thick walled and lignified. However, it also shows the beginnings of many features which are associated with the more advanced cacti: it has an enlarged, palisade cortex, a system of cortical vascular bundles, and medullary bundles. In addition, the wood of the uppermost portions of many branches is very parenchymatous, unlike the fibrous wood of the trunk. In some portions of the shoot, especially in the cephalia, the wood consists of just parenchyma and vessels, a trait which had been considered quite advanced. It seems reasonable that a strongly dimorphic plant such as Leptocereus would be able to give rise to the numerous diverse lines of evolution which exist in the Cactoideae.