The problem to be attacked in this paper is suggested in the following passage from L. Cockayne (“Notes on the Plant Covering of Kennedy's Bush, and other Scenic Reserves of the Port Hills,” Report on Scenery Preservation, Parliamentary Paper C.-6, 1915) concerning S. saxifragoides: “It also is a most striking plant. Now, an almost identical species, named Senecio lagopus, also occurs on the main mass of Banks Peninsula, which differs from S. saxifragoides merely in the possession of numerous bristles on the leaf, whereas in the latter such are absent. Yet, so far as is known, S, lagopus does not occur on the Port Hills, nor S. saxifragoides on Banks Peninsula proper. If this is truly a fact, the distribution of these two species, each equally well suited to the rock-conditions of the area, is one of the most remarkable cases of plant-distribution in the world.”
The same authority, in his description of his new species, Senecio southlandicus (Trans. N.Z. Inst., vol. 47, p. 118, 1915), further says, “The species is, indeed, far more distinct from S. bellidioides and S. lagopus than are these from one another. The classification of the whole series, including those already mentioned, together with S. saxifragoides Hook. f. and S. Haastii Hook. f., is in a most unsatisfactory position. Specimens are constantly coming to me from various correspondents which it is impossible to place with any degree of satisfaction. There are undoubtedly a number of well-marked forms, which demand, at the least, varietal names. Even one fixed character may serve quite well as a specific mark. This is illustrated in the case of S. saxifragoides and S. lagopus (the type from Akaroa), where the presence of numerous bristles, or their absence, on the upper surface of the leaf is the sole distinguishing character, so that, so far' as large plants of the two are concerned, if this character were not present no one could consider them in any degree different.”
In this paper attention has been directed to these two species solely as they occur on Banks Peninsula.
Banks Peninsula is situated in lat. 43° 32′ S. and long. 175° 30′ E., and forms a rough elliptical salient on the east coast of the South Island of New Zealand. Its diameter in a N.W.—S.E. direction is about twenty-five miles, and its breadth at right angles thereto about eighteen miles. Some forty miles to the westward stretches the main chain of the Southern Alps, from which the peninsula is separated by the gently inclined expanse of the Canterbury Plains, so that it is almost as completely isolated as regards the distribution of subalpine vegetation as if it had been separated from the mountain region by the sea.
The oldest rocks within its limits consist of Trias-Jura sedimentaries overlain in places by a thin veneer of Cretaceous rhyolites, but the main
mass of the peninsula was built up in mid-Tertiary times by flows of basalt and fragmentary material of similar lithological character, poured out from two vents situated somewhere near the centres of Akaroa and Lyttelton Harbours. A third focus of activity lay near Mount Herbert (3,012 ft.), but it was of less importance, although it was responsible for the formation of the highest peak in the area. The high cones thus formed were subject to paroxysmal explosions of moderate intensity, and their surface was modified by the establishment on their outer slopes of a well-developed system of radiating valleys. Volcanic action ceased in all probability long before the end of the Tertiary era. After the stream-system had reached a mature stage the land sank, and the sea entered the floors of the enlarged craters and extended a considerable distance up the lower reaches of the valleys, and these now form marked indentations of the coast-line. Owing to the prolonged weathering the land is now covered with a rich and fertile soil, and steep rock-faces occur only on the coast and at higher levels, where the more resistant basalts form at times precipitous cliffs—the' characteristic habitat of the senecios under consideration.
Map of Banks Peninsula and Port Hills, showing distribution of the two species of Senecio. L, Senecio lagopus; S, Senecio saxifragoides A, rhyolite escarpment where S. lagopus occurs; B, rhyolite escarpment where neither species occurs.
The following are the most important geological considerations affecting the distribution and ecological conditions of plants established in the locality:—
The isolation of the region from neighbouring mountain areas since it was first formed.
The uniformity of the lavas which form the majority of rocks in the area. No anomalies of distribution can be interpreted in the light of lithological differences in these rocks.
As a result of prolonged denudation the crater-ring of the Lyttelton volcano has been broken down at its south-western side and a sector completely removed, so that a stretch of comparatively low country, nowhere over 875 ft. in height, and consisting of exposed rhyolites and sedimentaries, separates the northern part of the crater-ring from the other part of the peninsula. This northern part forms the low range usually called the “Port Hills,” and is referred to throughout this paper, as the habitat of Senecio saxifragoides, by this name.
For a fuller account of the geological features of this area see J. von Haast, Geology of Canterbury and Westland, 1879, and R. Speight, “The Geology of Banks Peninsula” (Trans. N.Z. Inst., vol. 49, pp. 365–92, 1917).