The form of the Malvaceæ[87] and Bombaceæ, represented by _Ceiba_, _Cavanillesia_, and the Mexican hand tree (_Cheirostemon_), has immensely thick stems, with lanuginous, large, cordate, or indented leaves, and magnificent flowers, frequently of a purple-red. To this group belongs the Baobab, or monkey bread-tree, _Adansonia digitata_, which, with a moderate height, has occasionally a diameter of 32 feet,[JB] and may probably be regarded as at once the largest and most ancient organic memorial of our planet. The Malvaceæ already begin to impart to the vegetation of Italy a peculiarly southern character. The temperate zone in our old continent unfortunately is wholly devoid of the delicately pinnate Mimosas[88], whose predominating forms are _Acacia_, _Desmanthus_, _Gleditschia_, _Porleria_, and _Tamarindus_. This beautiful form occurs in the United States of North America, where, under equal parallels of latitude, vegetation is more varied and luxuriant than in Europe. The Mimosas are generally characterised, like the Italian pine, by an umbellate expansion of their branches. An extremely picturesque effect is produced by the deep blue of a tropical sky gleaming through the delicate tracery of their foliage. Heaths[89], which more especially belong to an African group of plants, include, according to physiognomic character and general appearance, the Epacrideæ and Diosmeæ, many Proteaceæ, and the Australian Acacias, which have no leaves but mere flattened petioles (phyllodia). This group bears some resemblance to acicular-leaved forms, with which it contrasts the more gracefully by the abundance of its campanulate blossoms. The arborescent heaths, like some few other African plants, extend as far as the northern shores of the Mediterranean. They adorn the plains of Italy, and the Cistus groves of southern Spain, but I have nowhere seen them growing more luxuriantly than on the declivities of the Peak of Teyde at Teneriffe. In the countries bordering on the Baltic, and further northward, the appearance of this form of plants is regarded with apprehension, as the precursor of drought and barrenness. Our heaths, _Erica (Calluna) vulgaris_, and _Erica tetralix_, _E. carnea_ and _E. cinerea_, are social plants, against whose extension agricultural nations have contended for centuries, with but little success. It is singular that the principal representative of this family should be peculiar to one side of our planet alone. There is only one of the three hundred known species of _Erica_ to be met with in the new continent, from Pennsylvania and Labrador to Nootka Sound and Alaschka.