Foweraker. — Mat- plants and Cushion-plants of Cass- River Bed. 7 



lower grades of transition terrace has not the power of retaining much 

 moisture, and the area becomes practically a desert for the time being. 



From the matter of the preceding paragraphs it may be concluded that 

 the climate varies from hygrophytic to xerophytic. Though considerable 

 rain falls, still there are periods, as mentioned above, when the " river-bed " 

 especially experiences desert conditions ; hence the need for the plants to 

 have a xerophytic structure- — a character illustrated in nearly all the species 

 dealt with. 



IV. AlJTECOLOGY OF THE ClTSHTON-PLANTS. 



(A.) Rao i dia tenuicaidis. (Plate III, fig. 1.) 

 (a.) Habitat. 



This is the dominant plant of the transition terrace in its earlier stages. 

 It is one of the first plants to establish itself on the first grade of terrace, 

 and its greenish or greyish mats, perhaps 1 m. in diameter, form the chief 

 element of the scanty plant-covering found thereon. The mats, later on, 

 frequently run together so that large patches, 25 square metres in area, 

 become covered with this plant. 



On the second or third grade of transition terrace R. tenuicaulis frequently 

 grows on the margins or the " banks " of the terrace, and hangs down over 

 the edge, thus helping to consolidate the latter and to resist the effects of 

 weathering and river erosion. (Plate III, fig. 2.) It frequently forms the 

 chief covering of various " islands " that stand up above the level of the 

 general shingle of the lower grades of terrace. In these cases it is evident 

 that the plant has had a consolidating effect, and holds together the area of 

 which it forms the covering. But, as Cockayne and Foweraker have stated, 

 these areas are larger than on many river-beds, owing to the width of that 

 of the Cass in relation to the flow of water (1916, p. 176). Owing to the 

 slight difference in coloration of the various mats on such an area (see 

 *" Coloration " below), the individual outline of each mat can be made out. 

 Besides river-bed, R. tenuicaidis frequently colonizes other bare shingle 

 — e.g., railway embankments and shingle laid bare by denudation. Also, 

 it can occupy denuded clayey soil and rock. 



(/3.) Life-form. 



(1.) General. — This species forms a bright-green flat mat, roughly circular 

 or ellipsoid (Plate III, fig. 1), compact in the centre, but very loose and open 

 at the margin. The surface is rough ; it has a " bristly " appearance, 

 owing to the numerous acute leaves, but owing to the laxness of growth 

 and the flexibility of the leaves and stems the general " feel " of the mat 

 is soft. The general appearance of the surface of the mat is not quite uni- 

 form, owing to the laxness of the marginal growth ; moreover, the leaves 

 of the new growth are often larger and more hairy than the older ones, and 

 this aids in the contrast. There is a considerable resemblance between 

 the leaves on new growth and those on young plants, except that the new 

 leaves are not quite so large as those on the juvenile form. ' This species is 

 of much more rapid growth than the other cushion -plants. The marginal 

 branches spread rapidly over the shingle and are closely applied to it, their 

 tips being slightly ascending. The mats differ greatly in size. Three that 

 were measured had the following dimensions: 15 cm. by 12 cm., 70 cm. 

 by 70 cm., and 130 cm. by 55 cm. The growth is centrifugal. The stems 

 lie close to the substratum, and give off copiously the terminal branchlets 

 above and very numerous roots below. 



