154 THE INSTABILITY OF LEAF-MOKPHOLOGY, 



cannot be maintained, when the supply of water is exhausted, 

 the leafage, as a consequence, becoming attenuated. Hetero- 

 phylly, and dimorphism, may (in part) be attributed to this cause. 



llydrophyUy. — The degree of permanence of the watei'-supply 

 (river, creek, lagoon, waterhole, &c. ) necessitates elasticity in the 

 leaf-characters of the individuals, or associations of plants fre- 

 quenting these stations. 



Instability of foothold. - One of the devices adapted as a pro- 

 tection against uprooting, by plants growing in the shallow 

 pockets of soil in alpine situations, and exposed to fierce storm- 

 blasts, is the resetted form of basal leaf (16; p. 3 3). This char- 

 acter is simulated by several swamp-dwellers, notably members 

 of the Orders Goodeniacese and Droseracese, as a supporting 

 agency in maintaining an upright position, and to counteract the 

 laxity of the soil-conditions obtaining in a bog. An instance of 

 a plant {Goodenia dimor-pha Maiden &. Betche) which had been 

 prostrated by a storm, growing an extra, basal rosette on a branch 

 which touched the ground, and rooting it to secure an anchorage, 

 was given (13; 1914, p. 470). The muddy, insecure environment 

 of the saline estuary has imposed on its plant-guests the task of 

 making provision for repelling tidal invasion; and similarity of 

 leaf-characters has been brought about, througli the use of the 

 same protective devices, by Zoysia pungens Willd., and Sporo- 

 bolus virgirdcus Humb. <k Kunth, the convolute leaves of these 

 estuarine grasses offering a minimum of resistance to the ebb and 

 flow of the tide. The insecurity of the shifting sand-dune is, in 

 some measure, responsible for similarity in the foliage of the 

 carpet-forming species, Mesembryanthemiivi edule Linn., and M. 

 cequilaterale Haw., though, in this, as in the previous examples, 

 more potent factors than those indicated have also been engaged 

 in moulding the leaf-characters of these plants. 



Drainage. — Stagnant water lying at the roots of plants in 

 clayey soils, clay-pans, and shallow rock-basins, appreciably 

 alters the facies of the foliage. In hilly sandstone-country, 

 sudden dessication, owing to rapid drainage after a copious 

 rainfall, compels the dweller in such regions to modify the size 

 and texture of its leaves. 



