Foweraker. — Mat-plant* and Cushion-plants of Cass River Bed. 19 



(8.) Epharmonic Variations. 



Raoulia lutescens is an exceedingly stable type. In the Cass Valley 

 careful examination of many cushions on grades 2 and 3 terrace showed 

 no perceptible difference so long as they were well exposed. In a few in- 

 stances cushions were found growing in the shade of large Discaria bushes 

 and of Festuca tussocks ; these showed a laxity of growth, with longer 

 branchlets and greener leaves than usual. Such cushions were dead in many 

 places, and the remaining portion appeared to be having a losing struggle 

 -with the quicker-growing herbaceous plants in its neighbourhood. Plants 

 wrown for six months in a moist greenhouse at sea-level showed a great 

 elongation of the branchlets. a laxity of growth, and a production of larger, 

 thinner, less hairy and consequently greener leaves. 



(€.) Relation to other Plants. 



As stated already, the typical habitat is on terrace grades 2 and 3; 

 but as this form of terrace merges into more consolidated forms different 

 edaphic and environmental conditions arise. R. lutescens is accustomed to 

 creep over stones and sand, but plants have been observed among tussocks 

 and Discaria toumatou, when the branchlets, more especially the marginal 

 ones, elongate and become greener. The same occurs where a tussock 

 throws up through the middle of a cushion. These branchlets in contact 

 with the tussock elongate considerably, but soon die off. On the other 

 hand, the marginal branches can climb up the face of a boulder and live. 

 It would seem that a sohd substratum is necessary for the establishment 

 of the cushion. 



Few cushions of R. lutescens. exist on the margins of the old terrace, 

 for there the plant rapidly succumbs before the advance of other vege- 

 tation. Its cushion, which in the older plants is of considerable depth 

 (5 cm. in some instances), affords a convenient growth-bed for other plants. 

 Foreign roots can penetrate among its branchlets and make use of the 

 nutrient filling-material. Surrounding plants encroach on its margins, and 

 a whole plant-community takes possession of the cushion. Such invaders 

 rapidly spread. Their growth deprives the cushion of air and light; and 

 though in some places it makes a strenuous effort to lengthen its branchlets, 

 nevertheless it is doomed, and sooner or later dies completely away, its 

 sole remnant being a rich mass of humus at the feet of its conquerors. 



One cushion (area, 05 square metre) growing at the exterior edge of 

 grade 3 transition terrace had a large plant-community upon it. 



But it is not alone with non-cushion plants that our types have to strive. 

 They often intermingle with one another, and an interesting piece of work 

 would be to watch the struggle for supremacy. 



Seldom does R. lutescens occur on the higher terrace grades in large 

 pure patches ; it is usually broken up and divided. One of its chief enemies 

 is the xerophytic moss Racromitrium lanuginosum, which rapidly encroaches 

 on its margins and reduces its cushions to a bed of humus. 



(£.) Conclusions. 



R. lutescens exhibits in its growth-form and structure all the character- 

 istics of a xerophyte- — e.g., very compact, low-growing cushion; small, 

 coriaceous, densely hairy leaves with aqueous tissue ; extreme capillarity 

 of cushion ; well-developed endodermis of stems ; and capacity for growing 

 in a habitat devoid of surface humus. 



