Vegetative Habit

The most pronounced trend in the evolution of Vegetative Habit is from the terrestrial to the epiphytic. the tribes Apostasicae, Cypripedieae, Neottieae and Orchideae are alll terrestrial. The primitive members of Epidendreae like Arethusinae, Gastrodiinae, Pogoniinae etc. are also terrestrial while the advanced forms like Oncidiinae and Sarcanthinae are wholly epiphytic in habit. With this transition, there are also attendant changes, as for example, from fleshy soil roots to spongy velamen roots. The primitive forms retain the typical monocotyledonous types of leaves - many in number, scattered or arranged spirally al along the length of the stem and plicate. this is seen is Apostasia, Selenipedium, Tropidia, Cephalanthera etc. In the advanced forms the leaves are limited in number, arranged in a cluster or distichously and are conduplicate.

There is also a transition from the non - articulate and persistent type to the articulate and deciduous.

According to Dressler and Dodson, there are two basic patterns of evolution of pseudobulbs in Orchidaceae. In subtribes like Bletiinae ( Acanthephippium, Calanthe, Ipsea, Pachystoma, Phaius, etc.) Cyrtopodiinae ( Eulophia, Geodorum etc) Cymbidiinae, Oncidiinae etc. the pseudobulb appears to have been derived from corm-like structures as in Bletia and Phaius. Here the pseudobulbs are comprised of a single internode. The second type is derived by the thickening of the entire stem as is seen in some Dendrobiums, Eria and Cattleya. In this group, a tendency exists to attain the uninodal pseudobulbous condition as in Dendrobium nanum, somespecies of Cattleya, Bulbophyllum etc.

A further tendency in the evolution of the stem is towards the attainment of monopodial from sympodial growth. This has been achieved along different phyletic lines. The genus Vanilla, which is primitive in several characters like nature of pollen, structure of the seed coat, etc, nevertheless, has terminal unlimited growth. A few genera in the subtribes Maxillariinae and Oncidiinae also have this type of growth. In the sarcanthinae this has become the rule.