Foweraker. — Mat -plants and Cushion-plants of Cass* River Bed . 33 



((3.) Life-form. 



(1.) General. — This plant forms a true cushion, especially when growing 

 on a substratum with a very open plant-covering, where it forms convex 

 cushions of varying sizes, and depths up to 12 cm. The contour is regular ; 

 the surface is rough owing to the subulate leaves. This plant differs from 

 those previously considered in possessing a very conspicuous long, stout, 

 central tap-root, and on a sandy or shingly substratum the adventitious 

 rootlets put forth from the horizontal stems are very few. In such 

 situations the whole cushion is remarkably loose when uprooted, and its 

 branch-lets do not adhere together as, for example, in Raoulia Haastii or 

 R. lutescens. The cushion is not tightly compacted, and is easy of penetra- 

 tion. On moist parts of the terrace, however, the cushions are flatter, 

 there are copious adventitious roots, and the margin of the cushion gradu- 

 ally merges into the surrounding low herbage. In fact, its growth-form 

 in such situations is almost a mat. 



(2.) Filling -material. — Visible filling-material does not appear till about 

 half-way down the cushion . The leaves on the branchlets below the rosettes 

 die and become a straw colour, but are slow to decay, and for at least the 

 depth of the branchlets no free filling-material occurs. Lower down, among 

 the older branches, appears humus, which is dark-coloured on the old 

 terrace, but fighter and containing much sand on the river-bed. As stated 

 above, the whole cushion is remarkably loose, and the single-rooted river- 

 bed forms of this plant can be uprooted, inverted, and have all their filling- 

 material shaken out. On old terrace filling-material appears sooner, and 

 quickly merges to a dense black humus, in which the lower stems are buried ; 

 indeed, in such situations the dividing-line between filling-humus and the 

 actual soil of the terrace is indistinguishable. 



(3.) Coloration. — The spring and summer colour of the cushion is a light 

 green with a faint tinge of yellow. In late autumn and winter the colour 

 is a brownish yellow. The causes of this coloration are discussed under 

 " Leaf-anatomy." 



(4.) Morphology. — (a.) Stem. — The main stems are prostrate, and radiate 

 from the top of the large central tap-root ; on river-bed substrata they 

 lie on the surface, but on old terrace they are more or less buried in the 

 surface soil. They have practically the same mode of growth and branching 

 as in some Raoulia forms, but the stems lack the rigidity of the latter. 

 They are perfectly supple, being in fact as supple as solid rubber : the 

 cause of this is explained by their anatomy. They are of a pale-straw 

 colour; the upper branches are clothed more or less with the remains of 

 the dead leaves, which do not decay away so rapidly as in the raoulias. 



The branchlets, with the leaves, are about 4 mm. in diameter, but the 

 leaves (1 cm. long) are subulate and in opposite not very close pairs. The 

 branchlets are lightly compacted in the cushion, but have no coherence. 



Transverse sections of a young stem show the following anatomical 

 structure : Regular epidermis with well-developed cuticle ; cortex of 

 spheroidal parenchyma bounded on the inner side by an endodermis with 

 thick walls, giving a dark-brown reaction with chlor-zinc-iodine, and pro- 

 bably strongly suberized ; pericycle of several layers of parenchymatous cells ; 

 vascular bundles of normal structure ; and a small parenchymatous pith . 



Regarding the anatomy of an old stem the following points of import- 

 ance occur : A phellogen appears in the pericycle, which forms layers of 

 tabular cork cells, with the result that the endodermis and cortex are 

 thrown off. The walls of the pericycle cells within the phellogen become 



2— Trans. 



