stages. This naturally follows, for the under surface is now the more exposed. In the bundle we find a reduction of elements corresponding to the reduction in size, but there are still large groups of transfusion tracheids at the sides of the bundle. Transverse Section: Base where Leaf has joined Stem. Stomata We find no stomata now on the upper surface, for the region in which they occurred has become joined to the stem. The stomata are then, on the final stage, only on the lower surface, and are here on the exposed surface; but they are greatly sunk, and are protected by the very close adpression of the leaf to the stem, and by the overlapping of neighbouring leaves. It is hardly necessary to give a summary of this leaf, the description being scarcely more, but it may be as well to mention again that (1.) In Dacrydium Kirkii we have an example of dimorphic foliage in a different genus to that of kahikatea. This dimorphic foliage, however, occurs only on the old plants, while in kahikatea it occurs only in the younger stages. The dimorphic foliage in D. Kirkii was a result of reduction from the more primitive form; that of kahikatea was the result of an enlargement of this form. (2.) In this leaf we have an example of stomata preserved on both surfaces of a broad leaf to the mature stage. Stomata at this stage were absent from the broad leaves of totara, miro, and matai. The presence of these stomata, and the absence of a sclerenchymatous hypoderm, makes it possible to explain why a reduction has taken place in this species. COMPARISON OF DIFFERENT FORMS OF LEAVES. The species I have chosen represent very fairly the different types of foliage found in the New Zealand Podocarpeœ; but, as my thesis is already very extensive, I shall not be able to give at present a comparison of these species with the other forms. I should like to add, however, that the most common form of leaf in the New Zealand Podocarpea is that represented by totara, miro, matai, and the earlier stages of Dacrydium Kirkii. Of these species the totara-leaf represents the most advanced form of this type, miro and Dacrydium Kirkii the simplest, whilst matai is intermediate between the two. A comparison of the structure in the "broad lamina " leaves of the Podocarpeæ, in conjunction with their habitats, might lead to some very interesting phytogenetic considerations. The totara is obviously the best adapted for living in exposed positions, and it is found where miro and matai could not survive. This type of foliage, which, in many respects, corresponds to Taxus baccata, is supposed to represent the most primitive type of Conifer leaf. The prevalence of this type in New Zealand Conifers is very suggestive when we consider the complete isolation of New Zealand from other countries, an isolation which can only have taken place at a very early geological period. Very different from the first type of foliage are the reduced forms also found in the New Zealand Podocarpea. The reduction in Dacrydium Kirkii is a later development in its life-history, but in rimu and kahikatea we find from the beginning of development very much reduced forms. This reduction incites, both in kahikatea and in rimu, an attempt, though very different in each, to increase the surface for assimilation in the young plants. It is very probable that this reduced form may have been derived through scale leaves like those of the mature Dacrydium Kirkii, but it is not within the scope of this thesis to go into phytogenetic details regarding the origin of the different types of Conifer foliage. It is hardly necessary here to draw any further conclusions as regards the anatomical development in these species, as I have given summaries and comparisons as I have proceeded. My investigations have not been extensive enough to draw many general conclusions for the whole group, but I should like to show before concluding how far the development in these species agrees with that of the Abietinea. For this purpose I will give a very short summary and comparison on parallel lines to that of M. Daguillon, which is quoted in the introduction of this thesis. 1. In the Podocarpeæ, as in the Abietinea, the existence of leaves intermediate between the cotyledon and mature leaves is constant. 2. The passage from the primordial form in all species investigated shows insensible transitions. We find nothing to compare with Pinus, for though in the two plants with dimorphic foliage-Podocarpus dacrydioides and Dacrydium Kirkii-we find often abrupt changes, insensible transitional forms also occur. 3. In the Podocarpea, as in the Abietineæ, the passage is sometimes marked by a modification of phyllotaxis-e.g., totara. 4. Sometimes by a change in the epidermal surface. This change is perhaps more marked in species of the Podocarpeœ than in the Abietinea. One or two parallel changes occur in species of the two groups, especially as regards the position of stomata. 5. In both groups there is a development below the epiderm of a sclerenchymatous hypoderm, though we find remarkable exceptions in the cases of miro, matai, and Dacrydium Kirkii. It might be noted here again the frequent occurrence in the Podocarpea of tannin-sacs in the layer next to the epidermis. Daguillon does not mention anything of the kind as occurring in the Abietineæ. 66 6. It is interesting to note the almost complete absence of pericyclic sclerenchyma" in the Podocarpeæ; one or two isolated fibres alone occur. The only strengthening development here is the row of sclerenchyma cells round the resin-canal. This must, however, form a very strong support for the leaf, owing to the arrangement of these cells in a circle. Daguillon also notes the presence of transfusion tissue in the pericycle, but its distribution is very different in the two groups. In the Abietinea it generally extends right around the bundle, often appearing to be connected with the phloem; in the Podocarpea this tissue generally occurs in groups at the sides of the bundle. From the position of the transfusion tracheids, as shown in Daguillon's figures, it seems more likely that they originated from the centrifugal than from the centripetal xylem. Daguillon himself says nothing about their origin, evidently regarding them as modified pericyclic cells. Tannin-sacs occur in the pericycle of many Podocarpeæ. 7. A bifurcation of the bundle like that occurring in the later stages of the Abietinea does not occur in the Podocarpeæ. The bundles of the mature leaves are, however, broken up by medullary rays. It is in the case of a cotyledon-i.e., that of totara-that we find the most parallel development. 8. In both groups the "number of conducting elements of the xylem and of the phloem augments when the primordial passes into the mature leaf." 9. In both groups also "when the parenchyma is heterogeneous and bifacial the differentiation of the palisade parenchyma is generally accentuated in the adult leaves." We see from this summary and comparison that in the Abietinea there are many anatomical developments similar to those we have noted in some of the Podocarpea. This similarity in development must not be confounded with the entirely different matter-similarity of structure. The leaves of the two groups are generally very different both in external form and in the arrangement of their component anatomical elements. But in both groups, to put the matter generally, disregarding all specific differences, the development tends to the differentiation of tissues for protection and strength, and also, both in the bundle and in the parenchyma, to modifications for increasing the power of conduction. To sum up in a few words: the development of the successive stages of Conifer leaves is, to a very great extent, merely the acquisition in the mature leaves of better appliances for the manufacture of food, and for its protection during the processes of assimilation. The following are transverse sections, unless otherwise stated: PLATE VIII. Fig. 1. Vascular bundle, youngest stage; totara cotyledon. × 192. Fig. 5. Vascular bundle, youngest leaf. × 192. × 192. x 192. : × 192. × 192. × 192. Fig. 6. Tangential section, young cotyledon; totara. Fig. 10. Transverse section, showing transitions in size of transfusion tracheids from pr to endodermis; shrub; totara. Fig. 11. Middle elements; older leaf, totara. Fig. 12. End of mature leaf, totara. × 100. × 100. × 100. Fig. 13. Outlines, transverse section (a) mature totara; (b) mature miro; (c) youngest leaf; (d) cotyledon (miro). x 12. Fig. 15. End of cotyledon; miro. Fig. 16. Tangential section, bundle, cotyledon; miro. × 164. Fig. 17. Bundle, older leaf, miro, stage 1. × 164. Fig. 19. Radial longitudinal section transfusion tissue; mature miro. Fig. 23. Bundle, flattened leaf, second year; kahikatea. Fig. 22. Bundle, awl-shaped leaf, second year; kahikatea. × 100. × 192. × 192. Fig. 24. Outlines, transverse sections-(a) cotyledon; (b) stage 1, awlshaped; (c) stage 1, flattened form; (d) leaf, awl-shaped, on plant three years old; (e) flattened form on three-year-old plant; (f, g, h) different mature forms; kahikatea. × 22. 72 PLATE X. Fig. 25. Flattened form, plant three years old, lower surface elements; kahikatea. × 192. Fig. 26. Flattened form, plant three years old, upper surface elements; kahikatea. × 192. Fig. 27. Bundle, mature kahikatea. × 192. Fig. 28. Rimu; bundle, cotyledon. x 192. Fig. 29. Transverse outlines-(a) cotyledon; (b) stage 1; (c) stage 2; (d) stage 3; (e) shrub; rimu. Fig. 30. Stage 1; rimu; bundle. × 22. × 192. × 192. × 192. x 192. Fig. 31. Stage 2; rimu. Fig. 34. Transverse outlines, Dacrydium Kirkii (a, b, c) sections from apex to base of mature scale leaf; (d) large form of leaf; (e) large form, natural size; (f) scale form, natural size. x 12. ART. IV. Some Observations on the Schists of Central Otago. Communicated by Dr. P. Marshall. [Read before the Otago Institute, 8th October, 1907.] 1. DENUDATION FORMS. MANY of the Central Otago ranges are capped by vast assemblages of rock hummocks or buttes. These are well displayed on the Dunstan, Old Man, Carrick, and Rough Ridge Ranges, also at Barewood and Macrae's. As we approach the coast these hummocks become more numerous and smaller, till they finally disappear. They are best seen near mature river development, while sufficient erosion removes them altogether. They are thus not enduring features of the landscape, but are brought into existence, and again destroyed, by erosive activity. These peculiar forms have been remarked by several observers, notably the late Captain Hutton* and Mr. T. A. Rickardt; but the only one who discusses their nature is Rickard, who studied them at Barewood. As he observes, they are generally composed of more siliceous and resistant portions of the rock. Basins and cavities are frequently developed near their base, * F. W. Hutton, "Geology of Otago " (Dunedin, 1875), p. 91. †T. A. Rickard, "Goldfields of Otago," Trans. Am. Inst. Min. Eng., vol. xxi, p. 411. |