Fowkrakkr. — Mat-plant* and Cushion -plants of Cass River Bed. 41 



apparently is the function of the pericycle stereome which forms a cylinder 

 round the phloem. 



(e.) Leaf-xerophily.- — The xerophytic characters of the leaves are obvious 

 in the many forms considered. As such features of a leaf are directed 

 chiefly towards checking transpiration, it seems well to sum up the various 

 methods of checking transpiration exhibited by the plants under con- 

 sideration : — (i.) Reduction in transpiring-surfaces : All the species have 

 small leaves, and, where the same species occupies different habitats, 

 the more xerophytic the habitat the smaller the leaf, (ii.) Vertical posi- 

 tion of leaves : Most of the leaves tend to assume a vertical position, 

 (iii.) Compacting of leaves : These are often closely appressed to the 

 axis of the branchlet, and the branchlets are closely compacted together, 

 (iv.) Anatomical modifications of the epidermis: (a) Well - developed 

 cuticle — e.g., Scleranthus ; (b) hairs — e.g., raoulias ; (c) position of stomata 

 —sunken in some species, (v.) Water-storage tissue — e.g., all the raoulias. 

 (vi.) Presence of oil- — e.g., Coprosma. (vii.) Few intercellular spaces. 



(/.) Water-storage. — One characteristic feature of the leaves of all the 

 raoulias is their central mass of aqueous tissue. This tissue consists of 

 large polygonal cells which exactly answer Haberlandt's description of 

 water-tissue (1914, p. 398). During the heavy rainfalls water can be 

 stored up in this central tissue, and during drought, when desert conditions 

 prevail on the river-bed, the stored water can be gradually given up to the 

 photosynthetic tissue. 



(g.) Coloration and the Role of Anthocyan. — Haberlandt (1914, p. 117) 

 explains how many evergreen leaves acquire a reddish colour in winter 

 owing to the formation of anthocyanin, in which case the chloroplasts 

 require special protection against the injurious action of light, because 

 no appreciable regeneration of chlorophyll takes place at the low tempera- 

 tures which prevail at that season. Again (I.e.. p. 42), he states, "Antho- 

 cyanin is also widely distributed as a constituent of the cell-sap in vegetative 

 organs, especially in leaves, where it in many cases probably acts as a light- 

 screen which prevents excessive illumination." The same author also 

 states (I.e., p. 118) that Stahl has shown that leaves with anthocyan became 

 1-5° to 1-82° C. warmer than leaves without it when placed 30 cm. from 

 a bat's-wing gas-flame. Stahl considered that this increased temperature 

 in the anthocyan-containing leaves would accelerate metabolism and trans- 

 location. 



Both the screen and the heating theory would seem to fit the case in the 

 cushion-plants of this paper. The plants are fully exposed ; they grow 

 where no shade is possible : thus the screening hypothesis is tenable. 

 Further, the cold of the Cass Valley during the winter would seem to demand 

 a heat-absorbing agent (anthocyan) in the leaves. 



In support of these contentions the reader is referred to the experiment 

 with Raoulia Haastii described on p. 21. No anythocyan to speak of was 

 developed ; the plant remained as green through the winter as when 

 removed from its natural habitat, while the original cushion from which it 

 was cut became, like it fellows on the river-bed, a deep chocolate-brown. 



But Haberlandt concludes (I.e., p. 118), " It must, in short, be admitted 

 that, in spite of numerous interesting detailed observations, the general 

 physiological and ecological significance of the presence of anthocyanin in 

 vegetative organs is still very obscure." Cowles (1911, p. 529) ends his 

 discussion of this subject in a still more disheartening manner : " Few of 

 the theories here mentioned are more than guesses, and it may be that the 



