are usually to be found in the true “damp” hollows. On the other hand, the general nature of the plant covering of the Oreti River hollows described above, except for the absence of halophytes, corresponds with that of moist rather than with that of dry dune hollows of other parts of New Zealand. The abundant presence of such species as Epilobium nerterioides, Hydrocotyle americana, and the species of Gunnera, may be especially emphasized with respect to this. The reason no doubt is to be found in the prevailingly cool and damp climate of Southland. Evaporation from the sand will be for the most part low, with the result that the surface mantle of loose dry sand will be as a general rule shallower than in the case of dunes in drier parts of New Zealand. This is clearly of importance to the plant life both on account of the fact that the first root of seedlings, and new roots formed from creeping stems, must get down through this mantle before they become anchored and begin to absorb, and also because the loose sand is liable to be blown and so has burying power. Transpiration from the subaerial parts of the dune species also will be, for the most part, low. It is undoubtedly for reasons of this nature, then, that certain “moisture-loving” species are able to take part in the succession on the Oreti River dune-hollows. However, there still remains the fact to be considered that these species will have to endure occasional periods, especially during the summer, of more severe dessication of their subaerial parts.

In dealing with the autecology of the dune-hollow species then, the factors of the environment to be regarded as of especial significance are the loose dry nature of the surface layer and the occasionally-strong evaporating power of the air. Our notes will therefore concern both the growth form of the species and also their ability to conserve water. Details should also have been included with respect to the seedling plants, if such had been available. It is obvious that the seedling stage is a very critical one in the establishment of a plant on a surface which is dry and liable to move. These details should concern not only the anatomical characters of the seedling, but also the season of the year at which germination takes place, the depth below the surface at which the germinating seed lies, and the rate of growth of the first root in getting down to the moist substratum. Without having actual observations on these points to bring forward, we surmise that in these Oreti River dune-hollows, under the climatic conditions prevailing, the establishment of the seedlings will not be so precarious, during at least the autumn, winter, and spring months, as in dry dune-hollows in other parts of New Zealand.

There are, as is well known, a number of leaf characters which are usually regarded as efficient in the retardation of transpiration from the leaves, or in the holding in the leaf of a considerable reservoir of water. Among the chief of these are a thick cuticle, sunken stomata, a tomentum, compactness of the inner tissues with

small air spaces, and an unusually large development of these inner leaf tissues with a correspondingly increased ability to hold water. It is, of course, very rarely that these characters are all present together in any one species, and external conditions which are adequately met in one species by a certain combination of leaf protecting characters may be met in another species by other characters. The dune species described below differ from one another considerably in the character of their leaves and other exposed parts.

The following anatomical and growth-form details relate, of course, to the species as they were actually growing in the dune-hollows described in this paper. We have selected for description those species which play an especially significant part in the succession.

(a) Raoulia australis var. albosericea.

In the youngest seedlings found it was evident that the first root penetrates down through the upper loose sand layer before the first foliage leaves have fully developed. Both the cotyledons and first foliage leaves are strongly tomentose, and correspond closely in structure with those formed later.

Growth of the above-ground part and development of the small compact cushion is slow, but root development is fast and extends far horizontally just below the loose surface layer. One plant measured had a cushion 2½ inches in diameter, with straggling wiry roots up to 3½ feet long. Neither the stem and branch system nor the roots are as efficient in sand-binding as are those of the true creepers with their more open and more branched mat form.

It is known that there are several distinct varieties of R. australis. Foweraker (3, p. 14) states that the species, as it occurs on lower river terraces in the Cass River bed, forms a flat, more or less open, quickly growing mat with straggling branches.

On the dunes the small cushions are frequently to be seen more or less buried by the sand. It is well-equipped for withstanding drying. The form and structure of the leaf is closely similar in several of the species of Raoulia examined by Foweraker (3). In these species there is a more or less well developed palisade tissue under both surfaces of the leaf (3, Fig. 3). The usual spongy mesophyll is replaced by a water storage tissue of large cells. Both surfaces of the leaf are densely covered with tomentum, the hairs having a characteristic form. The lamina is somewhat folded inward along both sides of the midrib on the adaxial side of the leaf and the leaves more or less closely imbricate over each other. R. australis var. albosericea possesses all of these characters (Fig. 10); in addition, the stomata on the outer, abaxial surface of the leaf are noticeably

Fig. 10.—Raoulia australis var. albosericea—Leaf in trans. sect.

sunken below the surface, and the cuticle is heavier on that surface than on the other. The roots are especially wiry and tough, this being due to the heavy groups of fibre which develop in the pericycle.

This species can be regarded then as well fitted, morphologically, to act as the first comer on the bare floor of a hollow. As has been mentioned above, however, it usually seems to play no part in preparing the way for the next comers, so that the succession may be said to begin properly with those species next to be described.

(b) Hydrocotyle americana var. heteromeria.

A creeping, open, mat-forming species, with the rhizome close beneath the surface. Roots are borne freely at each node. The leaf petioles are long, and by their elongation the lamina is kept above the surface of accumulating sand. The rhizomes grow rapidly and branch, and together with the petioles and roots are efficient in binding

the sand. When this species is accompanied by other creeping species, as is commonly the case, the anchoring of the sand is very effective.

The leaf is glabrous, the cuticle not specially thickened, and the stomata (which are present on both surfaces) lie flush with the surface. In these respects, then, the leaf could be described as of a mesophytic type. In its internal structure, however, it is not typically mesophytic (Fig. 11). The palisade tissue is three layers

Fig. 11.—Hydrocotyle americana var. heteromeria—Leaf in trans. sect.

of cells thick, and will hold a considerable amount of water, and the spongy mesophyll is compact with only small air spaces, so that air percolation through the mesophyll will be slow. It would seem that it is to these internal leaf characters that this usually moisture-loving species owes its ability to withstand dessication.

The rhizome has a relatively wide cortex which stores starch so abundantly that a white deposit of it soon collects on the bottom of a watch glass in which sections are placed. No doubt the cortex can also hold a supply of water.

(c) Epilobium nerterioides var. minimum.

A freely branching, quickly growing creeper, whose small sessile leaves lie flat on the sand surface. The stems are surface growing and copiously rooted. The rapid elongation of the branches ensures the continued life of the plant when the older parts are buried by the drifting sand.

The leaf has the same glabrous nature, unthickened cuticle, and unprotected stomata, as has that of the Hydrocotyle, and also, on the other hand, a correspondingly well developed palisade tissue and compact spongy mesophyll. Raphide sacs and mucilage cells are abundant, especially in the spongy mesophyll, the mucilage cells undoubtedly being effective in holding water. We cannot agree with Miss Pegg (4, p. 167) in regarding the leaf of this species as “strongly mesophytic.”

(d) Raoulia australis var. apice-nigra.

As in the other variety of this species described above, the seedlings show a rapid development of the root system. The mature growth form is an open mat, the creeping stems being usually buried, the short lateral leafy branches growing up erect to the surface, growth being fast enough to keep the leafy parts above the accumulating sand. The leaves are densely tomentose, but less imbricating than in the var. albosericea.

The leaf anatomy corresponds very closely with that of the other variety. The stem is strengthened with a continuous zone of cortical fibre, and patches of pericyclic fibre are also well developed. The root has the same remarkable development of pericyclic fibre as in the other variety. Thus both root and stem are well protected from drying and from mechanical injury by moving sand.

(e) Pimelea Lyallii.

This shrubby species usually occurs in inland hill country, and has a low sub-erect habit. On the Oreti River dunes its growth form is similar to that of P. arenaria, the flexible straggling stems and main branches being frequently buried to a depth of many feet, especially when growing on the sides of the dune hollows. The leafy parts of the plant rise less above the surface and are more mat-like in P. Lyallii than in the other species, but the power of rapid stem elongation under conditions of accumulating sand is noteworthy in both.

The toughness and flexibility of the stems is due largely to the remarkable development of fibre intermixed with the soft bast, the bast forming long wedge-shaped patches separated by the more compressible tissue of the medullary rays. There is also a well-developed periderm. The leaves are covered on their abaxial surface with long silky hairs, these being also present on the younger branchlets and terminal buds. This tomentum undoubtedly plays an important part in retarding transpiration and in protecting the buds from mechanical injury by blowing sand.

The mature leaf of P. Lyallii has a thick cuticle on both surfaces. The stomata are of a xerophytic type and are much sunk below the surface (Fig. 12). They are confined to the adaxial surface, although

Fig. 12.—Pimelea Lyallii—Leaf in trans. sect.

in P. arenaria they are present equally on both surfaces. The epidermal cells on both surfaces show the very peculiar gelatinisation of their inner walls which is characteristic of most of the genera in the family (5, p. 716), a feature which accords with the fact that the family as a whole is a dry habitat one. These mucilaginous cell walls will hold water very retentively. Both the spongy mesophyll and the palisade tissue are of a rather loose and open nature with plenty of air spaces, as Miss Pegg found also in P. arenaria (4, p. 164). On the whole, however, the leaf must be regarded as efficiently protected from excessive drying.

(f) Geranium sessiliflorum.

This species, like the last named, apparently is common on sand dunes only in Otago and Southland (2, p. 91). In its typical form there is a multicipital crown to the tap root lying at or just above the soil surface. In its dune form this develops as an elongated, much branched stem, bearing adventitious roots, and buried right up to the branch apices (Fig. 9). The plant is clearly able to keep pace with the accumulation of sand around it, and keeps its leaves above the surface. The stem and branches are stout and woody, and densely clothed in their upper parts with the bases of the old petioles and their large scaly stipules. In their lower parts they are worn clean of the leaf bases by the moving sand, but are further protected by a thick periderm. The deep growing woody tap root also has a well-developed periderm. In the dune form the leaves are only about one-third the diameter of those of the type.

This species on the dunes thus forms small patches, the branches of the stem effectively anchoring the sand around the plant. As it sometimes occurs in considerable numbers on the floor of hollows, it can be regarded as playing an important part there.

The leaf lamina and petiole carries a certain amount of silky tomentum, and, more important still, the branch apices are closely covered by the scaly stipules of the leaves, and are thus well protected from dessication and mechanical injury.

The leaf anatomy is closely similar to that of the Hydrocotyle and Epilobium described above.

(g) Gunnera Hamiltoni.

This species forms very dense, extensive patches. It has a stout creeping rhizome at the surface of the sand which branches copiously, bearing a clump of stout rigid leaves at each node (Fig. 8). The nodes are near enough together for the leaves to form a closely interlocking system, with the result that in the main patches the surface of the sand is nowhere visible and is completely protected from the wind. These patches are so dense that it is but rarely that any other species is to be seen growing in them. From the edge of the patch the rhizomes extend outwards very rapidly (Fig. 7).

The rhizome is up to 5 or 6 mms. thick. Starch is abundantly present in the very wide cortex and also in the pith, and no doubt these tissues will hold much water also. The roots are copiously provided with root hairs to such an extent that, when dug out, a cylinder of the moist sand firmly ensheaths each.

The leaf petiole is long so that the lamina is rarely buried by the sand. The lamina is thick and firm. The distinction between palisade and spongy mesophyll is not clearly marked, the cells of the former being scarcely at all elongated at right angles to the surface. There are, however, 3 or 4 layers under the abaxial epidermis, very compactly arranged, which represent the palisade. There is a considerable spongy mesophyll which gives the main thickness to the leaf. This also is compact, with but small air spaces. The stomata are on both surfaces and are not sunken. The cuticle on both surfaces is comparatively thin. Miss Pegg has described with a figure the leaf anatomy of G. arenaria (4, p. 168), this corresponding fairly closely with that of G. Hamiltoni. She speaks of the leaf as “strongly mesophytic.” This we cannot agree with, since the extensive and compact inner tissues will hold ample water, and air movements through them will be considerably retarded.

(h) Gentiana saxosa.

This is a herb of comparatively small size which can play the part of a true turf former. In some of the Oreti River dune hollows it occurs in large numbers, even on the sloping sides of hollows where the sand is loose and bare. Commonly all that is to be seen of the plant are the small oval leaves, ½ to 1 cm. long, lying flat on the surface. The numerous branches, buried in the sand, will tend to stabilise the surface layer.