40 Transactions. 



This cushion-form is apparently well fitted to the environment. Wind, 

 rain, frost, desiccation, and snow are the influences to which the above 

 plants are variously subjected, and their particular growth-form is per- 

 fectly adapted to resist the harmful influences of these conditions. 



(b.) The long tajj-root is conspicuous only in Scleranthus and Pinielea. 

 It exists in the other forms as well, but is so early reinforced by hosts 

 of adventitious roots that it does not reach a high state of development. 

 Similar tap-roots are characteristic of many plants growing among shingle. 

 It is not difficult to see the advantage and importance of such an organ. 

 On the. lower grades of terrace the water-holding capacity of the shingle is 

 low. As already noted, rain rapidly scaks away, and the moisture-content 

 largely depends on the depth of the water-table. Hence the value of a long 

 root is sufficiently obvious. Also the fact must be noted that both the 

 Pinielea and the Scleranthus, the former especially, are rather of open 

 growth ; in fact, the former possesses no filling-material whatsoever, and 

 has to depend upon the distant water-supply of the substratum. 



(c.) Filling-material, as already described in detail, exists definitely in 

 Raoidia tenuicaulis, R. lutescens, R. Haastii, and Scleranthus bifiorus. This 

 material, combined with the compactness of the cushion, forms a medium 

 which has considerable water-absorbing and water-holding capacity — i.e.. 

 the body of the cushion is really a sponge reservoir. The possession of such 

 a mass of absorbent material renders the plant more or less independent of 

 its substratum ; indeed, it really has an ecological station different from 

 its neighbours devoid of filling-material. The large cushions of R. Haastii 

 are quite moist inside, even though the shingle all round them is practi- 

 cally devoid of water for a considerable distance below the surface. The 

 branches of such a plant give, out copiously adventitious roots into the 

 filling-material, and it really is equivalent to a plant growing on a humus 

 substratum. 



(d.) Stem-structure.- — The most striking feature in the stem-anatomy 

 of the raoulias is the well-developed endodermis. Haberlandt (1914, 

 p. 373), quoting Schwendener, explains that climatic and edaphic con- 

 ditions react upon the structure of the endodermis, which is always specially 

 strengthened in the roots of lithophytes and steppe-plants, the endodermis 

 becoming thickened to quite an extraordinary extent, "evidently in adapta- 

 tion to the alternation of periods of abundant water-supply with severe 

 droughts." Further (p. 371), Haberlandt says, " By this means [i.e., 

 by the endodermis] the ventilating system of the cortex is permanently 

 shut off from that of the central cylinder, with the result that considerable 

 negative pressures can be maintained in the water-conducting channels." 

 If this view be accepted, the strongly developed endodermis of many of the 

 plants must be a structure of considerable importance in relation to the 

 edaphic conditions. Certainly the above opinion of Haberlandt refers to 

 the endodermis of roots, but the same should be true of stems. 



Another adaptational feature of the stem is the early bonification of 

 the whole central column of pith and wood. This would, appear to be a 

 structural arrangement favourable to the pushing of the stems forward over 

 and amongst the shingle and sand. Haberlandt (I.e., p. 184) says, " Those 

 rhizomes which serve to fix the plant in the ground agree with roots in 

 having their mechanical tissues united to form a stout axile tube or a solid 

 central strand." Besides being able to force its way among the shingle, 

 the stem also is adapted to resist the crushing influences of boulders and 

 other debris which are washed on top of it in times of flood. Such 



