BRIEFER ARTICLES. TOLERANCE OF DROUGHT BY NEAPOLITAN CLIFF FLORA. (WITH THREE FIGURES) THE writer has already made mention of several of the most abundant species on the cliffs in the vicinity of Naples. Some of these plants seemed to offer sufficient points of interest to be worthy of more detailed study, and a few notes in regard to their summer condition are here offered. At Pozzuoli, where most of these observations were made, the strip of fertile soil which skirts the beach is bounded on the landward side by cliffs, in many places quite vertical, rising to a height of thirty to more than a hundred meters. These cliffs are occasionally of trachyte, but most frequently of gray or yellowish tufa, which in softness and porosity closely resembles the softest brick used in interior construction by American. builders. Decomposed it makes a moderately rich and very warm soil. The chemical composition of two such tufas (from the little island of Vivara, 8.3 miles (13km) from Pozzuoli, of the same volcanic series and not greatly different age from the Pozzuoli deposits) is as follows: On account of the porosity and the vertical extent of these tufas, the soil water sinks to great depths, springs (except profound volcanic ones) are unknown, and the southerly faces of the cliffs during the summer months appear perfectly dry. During the summer of 1904 the maximum temperature at 2 P. M. observed in the superficial layer of soil on the little local deposits of weathered earth on the faces of cliffs, was 57° C. Occasionally the month of July is quite rainless. Some experiments on the amount of moisture contained in the material of the faces of tufa cliffs and walls gave the following results: 1904 Loss of moisture at 100° C., per cent. Aug. 15. Top of Roman pillar of brick and tufa, portion about ultimate rootlets of Medicago arborea 2. I Sept. 1. Earth on face of cliff about ultimate rootlets of Artemisia 10.6 Sept. 8. Earth on face of cliff about ultimate rootlets of Matthiola rupe- 13.4 Sept. 9. Earth on face of cliff about ultimate rootlets of Artemisia 5.4 Sept. 10. Tufa-like clay, surface of cliff, no vegetation except Sedum sp. 0.7 These determinations were made at a time when the autumn rains had not as yet set in, and the tufa and earth examined were nearly at their minimum as regards moisture-content. A few light sprinkles during late August and early September had not materially affected their condition. In the cliff-side formations about Pozzuoli the most important woody species, arranged roughly in the order of their abundance, are: 1, Artemisia arborescens, L.; 2, A. variabilis, Ten.; 3, Helichrysum rupestre, DC; 4, Inula viscosa, Ait.; 5, Spartium junceum, L.; 6, Medicago arborea, L.; 7, Opuntia Ficus-Indica, Mill.; 8, Mesembryanthemum acinaciforme, L.; 9, Matthiola rupestris, Guss. Number 5 of this list has been much discussed as a typical summer deciduous xerophytic shrub, and so it did not seem worth while to investigate further its equipment for resisting the difficulties of its environment. Numbers 7 and 8, the succulent members of the formation, are not indigenous and might better be studied in detail in their original habitats. Of the other members of this little flora it may be said that in general they have not the aspect of extreme xerophytes. It therefore seemed likely to be a profitable bit of work to look into the qualifications which enabled these six species to support the very high summer temperature to which they are exposed, with, in general, an inadequate supply of water. In external appearance the only obvious characteristic common to most of the shrubs considered is the tufted form of the plant. Helichrysum and Matthiola show this well in the shape of the plant as a whole (fig. 1) and the Artemisias in the shape of the separate branches. It seems probable that this mode of growth is of use in removing the foliage of the plants as far as possible from the intensely heated cliff surfaces. The plants in question show few adaptations for extraordinary collection or storage of water. The roots of the Artemisias are often twice or more than twice as long as their stems, but the other four species have comparatively short roots. In all six species the means of checking foliar transpiration are only moderately developed. Most of them during the rainy season transpire abundantly. The loss per hour per 100 square centimeters of leaf surface (reckoning lower surfaces only) for Matthiola is 661mg, Helichrysum 750mg, Medicago 1200mg, and Inula 1431mg, at a temperature of about 30° C., when the moderately xerophytic leaves of the olive lose 450mg per hour. The leaf areas for the finely dissected leaves of the Artemisias were difficult to calculate, so twigs of these were compared with olive twigs by weight, and the loss per gram per hour of both species of Artemisia was found to be about 190mg when the loss of an olive twig was 60mg. FIG. 1.-Helichrysum rupestre (left), Matthiola rupestris (right). May condition. Putting the means of checking transpiration in tabular form, they may be indicated as follows: No one of the nine cliff species enumerated is without some xerophytic characteristics, but it is noteworthy that only two of the non-succulent species have ample and obvious protection from injury by drought. The Spartium, leafless during the drier months and with thick wax-coated epidermis and sunken stomata, endures months of rainless heat without injury. The Medicago (figs. 2, 3) has a far less xerophytic aspect, but it flourishes not only on the nearly vertical surfaces of tufa cliffs, but also on the bare tops of ancient Roman walls and pillars. It is, indeed, the characteristic shrub of the meager flora of these latter localities, often at a height of 10-20m above the ground and far beyond the reach of any moisture from the soil. Its roots are short and scanty, and the plant contains no mechanism for special storage of water, but the complete shedding of the leaves in early summer renders the shrub secure against fatal desiccation afterward. Unfortunately I was not able to make as many determinations of the rate of transpiration of M. arborea under various conditions, as would have been desirable, so the following values are far from exact; still they may serve to explain the tenacity of life of the species. The total transpiration per hour at 30° C. of a twig 8cm long was about as follows: May 1, leafy twig in water. June 17 leafy twig in water September 14, leafless twig, not in water. 180 mg 120 70 In the first case above given, the leaves were in the height of their activity; May 14 they looked as green and fresh as at first, but had lost a little of their power to transpire. By June 17 all the leaves had turned yellowish and had taken up permanently their paraheliotropic position. On September 14 the twigs were (and had long been) entirely leafless and appeared rather destitute of moisture. The figures speak for themselves, and the final rate of transpiration, less than 3 per cent. of the maximum, 1 For these figures the author is indebted to Mrs. HERBERT S. JENNINGS. indicates an extremely high degree of adaptiveness to variations in temperature and water supply. The greatly lessened loss of water from the leafless twig of September 14 was certainly in great measure due to its comparatively desiccated condition at the beginning of the half-hour during which it was allowed to transpire. Twigs in vigorous leafy condition (May 14) lost about 7 per cent. as much water through the cortex alone as through leaves and cortex taken together. It is evident that Medicago arborea depends for protection against the excessive transpiration mainly upon its summerdeciduousness. This is shown not only by the lessened loss of water after defoliation, as above stated, but also by the fact that in damp soil, as under large trees of Quercus Ilex at Lake Fusaro, where during the larger part of the day in summer the illumination is only from 1 to 5 per cent. of the total, the leaves of this species are hardly at all deciduous during the summer. The aspect of these shade plants (fig. 2) is notably different from that of those growing on cliff sides. The former are much taller and relatively more slender, with leaves fewer, larger, and longer petioled. Twigs of shade plants gave about one and one-half times as much leaf area as those of equal length from sun plants. Leaves on individuals growing in deep shade were less numer ous than on those growing in FIG. 2.-Medicago arborea, shade form, grown in a situation with one to five per cent. illumination. XI. |