Foweraker. — Mat-plants and Cushion-plants of Cas* River Bed. 17 



As the successive layers of branchlets are added, and consequently the 

 depth of this cushion increases, adventitious roots are given out into the 

 filling-material. The cushions, when uninjured, are very compact, and 

 offer considerable resistance to mechanical force ; but when once com- 

 mencing to die away in parts, or when a portion of the cushion has been 

 cut out, separation of the stems and branchlets is rendered easy. Older 

 cushions are very " crumbly " in this respects — much more so than young 

 flat ones. In some positions, where the mat grows over a flat boulder, the 

 lower roots and stems die away and serve merely as a humus basis for 

 the upper part, which in such cases is very easily disintegrated. 



(2.) Filling-material. — The filling-material is not plentiful, its scantiness 

 being due to the very compact growth of the cushion. The branchlets 

 are so closely compacted together that not much space is left for foreign 

 material to collect. The humus formed by the plant from its own leaves 

 is scanty ; the leaves are very small, and form, when dead, but a slight 



C. E. F. del. 



Fig. 5. — Raoulia lutescens. Diagram of stems and branchlets in 

 a cushion. A, horizontal stem: 1. 2, 3, 4, successive 

 tiers of branchlets; 4, tier of terminal branchlets. 

 B, plane below which adventitious rootlets arise. 



blackish-brown coating round the branchlets. The greater part of the 

 filling-material consists of sand, which is most abundant when the plant 

 grows on the lower grades, where it is more exposed not only to wind- 

 blown sand, but also more especially to occasional inundation and its 

 consequent sand deposit. On the higher grades of terrace the sandy con- 

 stituent of the filling-material is not quite so predominant, and more humus 

 is apparent. Still, in no case is there any considerable amount of free 

 humus ; it consists merely of a brownish coating formed by the dead leaves 

 round each branchlet. Lower down in these old cushions the filling- 

 material becomes blacker. The filling-material is always of a porous con- 

 sistency, and, as the spaces between the branchlets are small, the total 

 capillarity (" Schwammwirkung," Schroter and Hauri) of such a growth- 

 forrn as R. lutescens must be considerable. 



(3.) Coloration. — During the summer the colour is glaucous, due to 

 greyish tomentum of the leaves. In winter the coloration is different, 

 but difficult to describe. The anthocyan at the edges of the greyish-green 

 leaves gives a slight brownish tint to these parts, and this, in conjunction 

 with the greyish tomentum, makes the general effect a grey with a tinge 



