Foweraker. — Mat-plants dud Cushion-plants of Cass River Bed. 43 



are erect, and have comparatively large broad leaves and rosettes. As 

 growth proceeds their branches become prostrate, their branchlets become 

 compacted together, and the true cushion or mat arises. May we not assume 

 here that ontogeny recapitulates phylogeny ? May we not suppose that 

 our cushion-plants have arisen from lax, broad-leaved, mesophytic forms ? 



We can easily perceive how the leaves of our raoulias could become 

 smaller and more closely appressed to the stem as xerophytic conditions 

 increased, but the difficult point is how the prostrate form arose. Why do 

 the main stems lie prostrate on the ground ? This question is partially 

 answered by observing some of the other plants on the river-bed. Let us 

 consider Discaria toumatou. Plants of this species on the shingle have a 

 stunted form with quite prostrate branches (Plate VI, fig. 1), the " espalier- 

 shape " of Warming (1909, p. 26). On the moist old terrace, plants of 

 apparently the same age are upright, this being the normal form (Plate VI, 

 fig. 2). Similarly, gorse (Ulex europaeus) assumes the espalier-shape on 

 shingle. Such forms as Helichrysum depressum are true espaliers. What 

 causes this espalier shape ? Wind ? Insolation ? Desiccation ? It can- 

 not be wind or the direct sun's heat — i.e., insolation — because both river-bed 

 and old terrace are subjected to equal amounts. It must be desiccation 

 — the influence of the dry shingle. Just how the shingle affects the plants 

 cannot be told. Warming (I.e.) says, " Probably the cause must be sought 

 in the difference of temperature of the air and soil at the time when the 

 shoots are developing." The writer has noticed the same habit among 

 weeds in a gravel-pit. Here many of the ordinary introduced weeds 

 that are more or less erect in normal situations take on the espalier habit. 

 Further than this it does not seem wise to speculate. 



VII. Literature consulted. 

 (A.) Special. 



Beauverd, G., 1910. Contribution a, l'Etude des Composees (Suite iv), 

 Bull, de la Societe Bot. de Geneve, p. 207-53. 



Cheeseman, T. F., 1906. Manual of the New Zealand Flora. 



Chilton, C, 1915. Notes from the Canterbury College Mountain Bio- 

 logical Station, Cass : No. 1, Introduction and General Description of 

 Station, Trans. N.Z. Inst., vol. 47, pp. 331-35. 



Cockayne, L., 1900. A Sketch of the Plant Geography of the Waimakariri 

 Biver Basin, considered chiefly from an (Ecological Point of View, Trans. 

 N.Z. Inst., vol. 32, pp. 95-136. 



- 1899, 1900, 1901. An Inquiry into the Seedling Forms of New Zea- 

 land Phanerogams and their Development, Trans. N.Z. Inst., vols. 31 

 32, 33. 



1904. A Botanical Excursion during Midwinter to the Southern 

 Islands of New Zealand, Trans. N.Z. hist., vol. 36, p. 225-333. 



- 1909. The Ecological Botany of the Subantarctic Islands of New 

 Zealand, The Subantarctic Islands of New Zealand, vol. 1, pp. 182-235. 



- 1910. New Zealand Plants and their Story. Wellington. 



1911. On the Peopling by Plants of the Subalpine River-bed of the 



Rakaia (Southern Alps of New Zealand), Trans. & Proc. Bot. Soc. Edin- 

 burgh, vol. 24, pt. iii, pp. 104-25. 



1912. Observations concerning Evolution, derived from Ecological 



Studies in New Zealand, Trans. N.Z. Inst., vol. 44, pp. 1-50. 



