Note 24 — PHYSIOGNOMY OF PLANTS.

p. 26.—“The Pothos-form, Aroideæ.”

Caladium and Pothos are exclusively forms of the tropical world; the species of Arum belong more to the temperate zone. Arum italicum, A. dracunculus, and A. tenuifolium, extend to Istria and Friuli. No Pothos has yet been discovered in Africa. India has some species of this genus (Pothos scandens and P. pinnata) which are less beautiful in their physiognomy, and less luxuriant in their growth, than the American species. We discovered a beautiful and truly arborescent member of the group of Aroideæ (Caladium arboreum) having stems from 16 to 21 English feet high, not far from the convent of Caripe, to the East of Cumanas. A very curious Caladium (Culcasia scandens) has been discovered by Beauvois in the kingdom of Benin. (Palisot de Beauvois, Flore d’Oware et de Benin, T. i. 1804, p. 4, pl. iii.) In the Pothos-form the parenchyma is sometimes so much extended that the surface of the leaf is interrupted by holes as in Calla pertusa (Kunth), and Dracontium pertusum (Jacquin), which we collected in the woods round Cumana. The Aroideæ first led attention to the remarkable phenomenon of the fever-heat, which in certain plants is sensible by the thermometer during the development of their inflorescence, and which is connected with a great and temporary increase of the absorption of oxygen from the atmosphere. Lamarck remarked in 1789 this increase of temperature at the time of flowering in Arum italicum. According to Hubert and Bory de St. Vincent the vital heat of Arum cordifolium in the Isle of France was found to rise to 35°and 39° Reaumur, (110°.6 and 119°.6 Fahr.) while the temperature of the surrounding air was only 15°.2 R. (66°.2 F.) Even in Europe, Becquerel and Breschet found as much as 17½° difference, Reaumur (39°.4 Fahr.) Dutrochet remarked a paroxysm, an alternate decrease and increase of vital heat, which appeared to reach a double maximum in the day. Théodore de Saussure observed analogous augmentations of temperature, though to a less amount, only from 0°.5 to 0°.8 of Reaumur’s scale (1°.15 to 1°.8 Fahr.), in plants belonging to other families; for example, in Bignonia radicans and Cucurbita pepo. In the latter plant the use of a very sensitive thermoscope shews that the increase of temperature is greater in the male than in the female plant. Dutrochet, who previous to his early death made such meritorious researches in physics and in vegetable physiology, found by means of thermo-magnetic multiplicators (Comptes rendus de l’Institut, T. viii. 1839, p. 454, T. ix. p. 614 and 781) an increase of vital heat from 0°.1 to 0°.3 Reaumur, (0°.25 to 0°.67 Fahr.) in several young plants (Euphorbia lathyris, Lilium candidum, Papaver somniferum), and even among funguses in several species of Agaricus and Lycoperdon. This vital heat disappeared at night, but was not prevented by placing the plants in the dark during the day-time.

A yet more striking physiognomic contrast than that of Casuarineæ, Needle trees, and the almost leafless Peruvian Colletias, with Aroideæ, is presented by the comparison of those types of the greatest contraction of the leafy organs with the Nymphæaceæ and Nelumboneæ. We find in these as in the Aroideæ, leaves, in which the cellular tissue forming their surface is extended to an extreme degree, supported on long fleshy succulent leaf-stalks; as in Nymphæa alba; N. lutea; N. thermalis (once called N. lotus, from the hot spring of Pezce near Groswardein, in Hungary); the species of Nelumbo; Euryale amazonica of Pöppig; and the Victoria Regina discovered in 1837 by Sir Robert Schomburgk in the River Berbice in British Guiana, and which is allied to the prickly Euryale, although, according to Lindley, a very different genus. The round leaves of this magnificent water plant are six feet in diameter, and are surrounded by turned up margins 3 to 5 inches high, light green inside, and bright crimson outside. The agreeably perfumed flowers, twenty or thirty blossoms of which may be seen at the same time within a small space, are white and rose coloured, 15 inches in diameter, and have many hundred petals. (Rob. Schomburgk, Reisen in Guiana und am Orinoko, 1841, S. 233.) Pöppig also gives to the leaves of his Euryale amazonica which he found near Tefe, as much as 5 feet 8 inches French, or 6 English feet, diameter. (Pöppig, Reise in Chile, Peru und auf dem Amazonenstrome, Bd. ii. 1836, S. 432.) If Euryale and Victoria are the genera which present the greatest extension in all dimensions of the parenchyma of the leaves, the greatest known dimensions of a flower belong to a parasitical Cytinea, the Rafflesia Arnoldi (R. Brown), discovered by Dr. Arnold in Sumatra, in 1818: it has a stemless flower of three English feet diameter, surrounded by large leaf-like scales. Fungus-like, it has an animal smell, resembling beef.