42 Transactions. 



red pigments are merely the indices of certain chemical activities that ; 1 1 

 quite without functional significance." 



(h.) The Effect of the Mat and Cushion Plants in Consolidating the Lower 

 Terraces. — An important role played by Raoulia tenuicaulis, R. lutescens, 

 and R. Haastii is apt to be overlooked at first- — viz., the formation of a 

 humus layer and the consolidation of the terrace. R. tenuicaulis, as stated 

 above (pp. 7, 8), covers large areas with a mass of contiguous mats which 

 increase in depth, more and more filling-material collects, and sooner or 

 later other plants begin to grow thereon. The rich humus-bed is an excellent 

 germinating-ground for various seeds, and soon such Raoidia areas support 

 a varied plant-community.* Later on the Raoulia commences to die out 

 in patches, thus contributing further to the depth of humus, and so the 

 surface of the shingle is consolidated, and becomes a fit habitation for other 

 plants. 



VI. Evolution of the Cushion-plants. 



Before concluding, some speculations regarding the evolution of cushion- 

 plants seem allowable. 



It is easy to see the suitability to the habitat of the cushion-form, 

 with its surface composed of the rosettes terminating the branchlets. It 

 can be seen, too, that the branchlet-rosette form is adopted by many foreign 

 alpine plants, such as saxifrages and gentians. In New Zealand, on sub- 

 alpine fell-field or herb-field, such plants as Celmisia spectabilis, C. coriacea, 

 and C. Armstrongii, composed of a mass of large rosettes, are abundant. 

 At higher altitudes are species having smaller rosettes and smaller leaves 

 {e.g., C. viscosa, C. Sinclairii, and C. Haastii). At greater altitudes still 

 occur species with still smaller leaves, still smaller rosettes, and still greater 

 compactness of growth-form (e.g., C. argentea, C. sessiliflora, and C. larici- 

 folia). 



A similar gradation occurs in Raoulia. This genus has much the same- 

 growth-form as Celmisia, but the plants are generally smaller in every 

 respect. We can trace a gradual transition from a large-leaved, lax, meso- 

 phytic form with large rosettes like R. glabra to a firmer form like R. sub- 

 sericea ; then to shingle-tolerating forms like R. australis ; and upwards 

 through such forms as R. lutescens and R. Haastii, until we arrive at a form 

 like R. eximia, a denizen of dry alpine and subalpine rocks, one of the famous 

 "vegetable sheep" of New Zealand, and one of the most remarkable 

 cushion-plants in the world. 



From a comparison of such forms we may conclude with Cockaynef 

 (1912, pp. 21, 22, and 1911, p. 119) that the cushion-plants must have arisen 

 from a mesophytic form with large leaves ; from this form variants or 

 mutants arose which were better adapted to dry habitats, and which sur- 

 vived while the others perished. This process of natural selection doubtless 

 went on till now we have the numerous species before us. 



If we accept the recapitulation hypothesis of certain evolutionists, the 

 proof of the above course of events lies in the ontogeny of the various species. 

 Let us recall the seedling stages of Raoulia tenuicaulis. The seedlings 



* See similar remarks by Cockayne regarding the " epiphytes," as he calls them, 

 upon a cushion of R. Haastii (1911, pp. 121-22). On one cushion seven different 

 species were noted. 



f Cockayne, however, appears to attribute the change of growth-form entirely to the 

 cumulative effect of xerophytic conditions, and allows nothing for mutations or 

 Darwinian variation. 



