Foweraker. — Mat-plants and Cushion-plants of Cass River Bed. 39 



vegetative means. Small branches are broken off during floods and washed 

 down-stream. Those which are stranded and partially covered with gravel 

 soon take root and produce a new mat. The chief characteristic of the 

 stems is their capacity to creep round and under stones ; they rarely creep 

 over large boulders. 



Acaena microphylla, in several varieties, is ubiquitous in the Cass Valley, 

 and, growing well on bare spots, it naturally invades the river-bed. The 

 stems are quite prostrate, branch copiously, sending up at intervals short 

 vertical branches which bear the leaves and later the flower-heads. The 

 stems are very tough and wiry, and creep with ease among the shingle. 

 Koots are sent down frequently which hold the plant firmly, so that it is 

 difficult to root up lengths of it. The leaves are about 4 cm. long, pinnate, 

 with three to six pairs of leaflets, and have the usual rosaceous form. They 

 have varying degrees of hairiness, and are usually membranous. They he 

 at varying angles, and form the uneven surface of the mat. The flowers 

 are in heads, and the fruiting-calyx is pyramidal, four-angled, often with a 

 bristle at each angle. Greenhouse cultures resulted in a form with longer, 

 larger, more erect, and less hairy leaves. 



The chief point to notice about the above two species is the fact that 

 when growing on river-bed they become more wiry, more compact, have 

 smaller leaves, and show anthocyan development- — in a word, they assume 

 xerophytic features. They have great capacity for spreading rapidly over 

 the shingle, and to this end their stems can burrow among and thrust aside 

 the shingle with ease. 



(V.) Chief Characteristics of the Cushion-plants. 



The brief survey of the Cass cushion-plants just given brings out clearly 

 certain points of ecological importance, such as their adaptations, the 

 extreme convergence which they exhibit, their winter coloration, and their 

 various habitats. In the descriptions of the different species certain 

 general characters of cushion-plants have been brought out, which are 

 discussed under the heads which immediately follow. 



(a.) The Cushion Growth-form.- — The fact of plants belonging to such 

 diverse families as Polygonaceae, Caryophyllaceae, Rosaceae, Thymelaeaceae, 

 Eubiaceae, and Compositae assuming the mat or cushion form illustrates in 

 a striking way the principle of convergence.* Considering such forms as the 

 raoulias, Scleranthus biflorus, and Pimelea prostrata var. repens, we find them 

 all assuming the same growth-form. In all the above there is (primarily, at 

 any rate) a main central root, from the top of which radiates a series of 

 copiously branching prostrate stems whose vertical ramifications, becoming 

 more and more closely packed together, finally end in the terminal branchlets. 

 These, clothed with leaves, are compacted together in varying degrees of 

 density, giving corresponding degrees of solidity to the cushion. Their 

 terminal buds and leaves constitute rosettes, which together go to form 

 the surface of the cushion. 



Not only do these different plants resemble each other in their shoot- 

 branching, but some possess filling-material into which penetrate a greater 

 or smaller number of adventitious rootlets. 



* Were all the New Zealand cushion-plants being considered, the number of families 

 would be much greater and the convergence far more pronounced. 



