THE FACTORS OF DISTRIBUTION 43 



altitude, and not only modifies the form of trees and 

 shrubs, but also exerts a marked influence in limiting 

 their altitudinal extension. Its action upon plant-life 

 is both direct or mechanical, and indirect or physiological, 

 the latter being the more important. The slope and 

 bending of trees, the killing of their branches when 

 lashed against one another on the windward side, and 

 the curious one-sided branching produced by the killing 

 of these shoots may all be of mechanical origin. The 

 desiccating action of wind, drying the soil, favouring 

 transpiration, and so drying the plant also, is, however, 

 probably the most important action of wind upon 

 vegetation. It will be intensified where the soil is cold, 

 so that the loss of water by transpiration is not readily 

 made up. It is this which is probably the chief cause 

 of the dwarfing (nanism) and adpressed or " espalier " 

 habit of many Arctic and Alpine shrubs, and the cushion- 

 like growth of herbaceous species in similar situations. 

 Bud-scales, investing hairs or wooliness, and the retention 

 of dead leaves or parts of leaves., as in many desert 

 grasses which retain the sheath when the blade is 

 withered, are means of checking this excessive transpira- 

 tion caused by drying winds. 



It is plants growing in exposed situations, such as 

 level grass-lands and swamps, and many of our tallest 

 trees, that depend frequently upon wind for the con- 

 veyance of their pollen to the female flower; and the 

 " precocious " development of such " anemophilous " or 

 wind -pollinated flowers, i.e. their maturation before the 

 leaves are put out, is a well-known adaptation to this 

 end. The case of the Kerguelen Island Cabbage 

 (Pringlea antiscorbutica R.Br.), a wind -pollinated member 

 of the order Crucijevce, most of which order are self- 

 pollinating or insect-pollinated plants, inhabiting, as it 

 does, bare island rocks destitute of flying insects, is 

 specially noteworthy. (Fig. i.) 



So, too, it is the plants of exposed situations that 

 depend upon wind for seed dispersal. It is among 

 steppe-plants that we find whole plants uprooted and 

 rolled along by the wind ; among tall trees that we 

 chiefly have winged fruits and seeds; though the 

 feathery pappus, or tuft of hairs, occurs alike in the 

 dandelion or thistle of the plains, in the willows (whether 

 dwarf and Alpine or not), and in the epiphytes, such as 



