IX Different growth types in tropical areas
Raunkiaer defined the plant-life forms according to where the vegetative buds would lie under a 50cm deep snow line. He distinguishes the phanerophytes corresponding to the trees, the chamaephytes to the shrubs, the hemicryptophyte to the herbaceous plants, the geophytes which have underground tubers and finally the therophyte corresponding to the annual species. This succession from trees to herbaceous is correlated to the temperatures: the colder the climate is, the more short the plants will tend to be. This can also be used in tropical areas where the limiting factor will be water and not temperature. In Savanas and deserts there will be many more shrubs and geophytes. Several other factors are also to be taken into account like the phylogeny of the taxa. Humbertia has a tree like habit and this is probably linked to the fact that it belongs to the Erycibeae tribe. Dichondraea which is also a distinct tribe only has herbaceous plants.
Convergence between the different growth forms is common. Shrubs are common in American savannas but also in Africa. The thorns of some Convolvulus species are also found in African species of Ipomoea.
Some species have a very unique growth form like Erycibe stenophylla, a rheophyte or Ipomoea leprieuri an annual or perennial depending on where it germinates (Perennial on rich soils but annual on bare rocks, like observed by Patrick Blanc).