Foweraker. — Mat-plants and Cushion-plants of Cats? River Bed. 5 



more consolidated appearance than the bed proper, the interstices between 

 the larger boulders being well rilled up with small gravel and sand. Such 

 transition terrace forms not only the banks of the bed proper in some places, 

 but also small islands between streams. 



The transition terrace, grade 2 (Plate II, fig. 1), has the boulders and the 

 shingle well consolidated. There is much sand, and the plants thereon 

 have already commenced to form humus. The whole surface is by no 

 means covered with vegetation, but the cushion- and mat-plants already 

 there serve to assist other plants in gaining a footing. This grade is covered 

 with water only in times of great flood. 



The transition terrace, grade 3, is reached when the whole shingle area 

 has a distinct plant-covering. There is much humus, which is interrupted 

 only where the boulders and shingle-patches project above the surface. 



The terrace proper (Plate II, fig. 2) is the type which forms the greater 

 part of the flood-plain ; it is entirely covered with plants. As it stretches 

 away to the bases of the surrounding hills its character changes — it becomes 

 more consolidated, the depth of humus becomes greater, and it supports 

 a more varied vegetation, which merges into low tussock grassland. 

 (Cockayne and Foweraker, 1916, pp. 172-73). 



III. The Ecological Conditions. 



(A.) Edaphic. • 



It will be seen from the above descriptions of the various grades of 

 terraces that the substratum in which the plants grow is composed in the 

 three lowest grades largely of stones of various sizes mixed with small quanti- 

 ties of sand and silt. "Such a soil," writes Cockayne (1911, p. Ill), "is 

 deficient in available nutritive salts, and in itself provides merely desert 

 conditions for plant-life no matter how frequent the downpour." As the 

 terraces merge from bed proper to higher grades the proportion of sand 

 and silt to stones becomes greater until there is " a thin coating of light 

 silky loam which, though far from being a fertile soil, is quite sufficient 

 to support, with the addition of the self-supplied humus, a closed forma- 

 tion of those plants which are provided with certain ' adaptations ' ' (I.e.). 

 This stony soil is necessarily extremely porous, and after rain it rapidly 

 dries, so that ordinarily its water-content is quite small. Still, there is 

 always a small amount of moisture in all but those parts composed solely 

 of huge boulders. The large stones on the surface assist in checking evapo- 

 ration. There is always plenty of underground water ; indeed, many of 

 the New Zealand shingle-bed rivers flow to a certain extent underground. 

 The depth of the water-table varies, but it is always nearer to the surface in 

 the lower grades. 



" How far," to quote from Cockayne again, " the heat from the sun 

 can penetrate into the soil I cannot say, but probably to no noticeable 

 depth, except between the chinks of the stones. These latter become so 

 strongly heated during a period of insolation that one can hardly bear to 

 touch them with the hand, and the reflected heat must be very considerable 

 so far as the ground plants are concerned. On the other hand, the stones 

 rapidly lose their heat when the sun's rays are obscured" (I.e., p. 112). 

 "As for aeration of the soil, that will be abundant. From the above 

 it is evident that a river-bed is a strongly xerophytic station, and that, 

 notwithstanding an abundant rainfall, the conditions primarily resemble 

 those of a desert " (I.e.). 



