184 CHAEA-CTEKISIICS OF THE lO^ES OF MOISTUEE. 



largest Algae in Koyal Sound are usually not cast upon the shore by 

 the waves, and I have almost been dependent upon grapples thrown 

 from the rocks for specimens of the more delicate forms. Polysiphonia 

 Sullivance and lUiyiiphlcea Gomardii are amongst the novelties. 



[From the first report of the naturalist attached to the Transit-of- 

 Yenus Expedition to Kerguelen's Island, December, 1874, the Kev. 

 A. E. Eaton — Frooeedings of the Royal Society, 1875, pp. 351-6.] 



ON THE BOTANICAL CHARACTERISTICS OE THE ZONES 



OE MOISTURE. 



Br J. G. Biker, E.L.S. 



As in speaking of heat we have to distinguish four groups of plants 

 which have a special constitution in respect of the amount of heat they 

 require, so in speaking of moisture we shall have to separate plants 

 into three groups according to their needs as regards aerial humidity. 

 These are 1st, XerojyJnlous plants, which can live in climates in which 

 the air habitually contains very little moisture ; 2nd, Hygrophilous 

 plants, which can only live in climates in which there is habitually a 

 great deal of atmospheric moisture; and 3rd, Noterophilous ^l-duts, 

 intermediate in constitution between the two other kinds. 



Broadly stated, the grand influence which the distribution of 

 moisture over the earth's surface exercises upon the distribution of 

 plants is that the earth is girdled round in and near the borders of the 

 two rainless zones, which run like a belt round the earth near the two 

 tropics, and separate the region of periodic rains from the region 

 of irregular rains, with two broad belts of country in which the 

 Xerophilous plants predominate more decidedly than they do in any 

 other part of the world, and that they run out from these belts into 

 the interior of the continents, both towards the Equator and the Poles, 

 avoiding the insular climates. 



The concomitants in plant-form of the Xerophilous type of con- 

 stitution are as follows : — In Dicotyledons — 1. Leaves becoming very 

 thick and fleshy, with pulpy inner and leathery outer layers, in which 

 the air-passages and stomata are few, and the cells either small or 

 their walls thickened by secondary deposits of cellulose, as shown in 

 Mesembryanthemum, Sedum, Cotyledon, and Sempervivum. 2. The 

 stem condensed into a single central unbranched barrel-shaped or top- 

 shaped mass, which is either leafless and armed with spines, as in 

 Mam mill aria, Echinocactus, and various Euphorbias ; or without spines, 

 and bearing fleshy or rigid leaves, as in Cycads, Welwitschia and Yitis 

 Bainerii, and Y. Macropus. 3 Branching, fleshy, or hard-stem types, 

 without proper leaves, but in which the main stems or petioles put on a 

 leafy appearance, as in Opuntia, Phyllocactus, Colletia, and the phyllo- 

 dineous Acacias. 4. Much-branching shrubs, with copious whip- 

 like branches without either leaves or prickles, as Retama, Ephedra, 

 Rhipsalis, Cassytha, and Euphorbia Tirucalli, 5. Much-branched 

 wiry herbs or shrubs, with an excessive development of prickle, as 



