284 PLANT-LIFE 



or enclosed in a sticky fruit in the former case 

 order that they may be dispersed by the gentlest breeze, 

 or in the latter that they may be carried by birds and 

 deposited on the branches of trees. Many Orchids are 

 epiphytic. The higher epiphytes are best developed 

 and most abundant in the tropics. The position taken 

 up by epiphytes has evident relation to the demand for 

 light, and their devices for conserving water are essential 

 for successful growth on the bark of a tree. It should 

 be added that epiphytes will grow in soil if the lighting 

 conditions are suitable. 



But the light demanded by plants must be just 

 enough, and not too much. A too great light intensity 

 may be just as deleterious as an insufficient illumination 

 is disastrous. The positions assumed by leaves to avoid 

 excessive transpiration (p. 266) may also serve to pre- 

 vent the injury resultant upon too full an exposure to 

 strong light. The Ferns which flourish in the shade of 

 rocks or on the floor of a wood are so well adapted to 

 their situations in relation to light that if they are sub- 

 jected to intenser light by the removal of the objects 

 which give them shade they lose their vigour, and become 

 pale and sickly. So is it with such a plant as the Wood- 

 ruff (Asperula odorata, Plate L.). This flourishes well 

 in shade, particularly that of a wood, but if the wood- 

 cutter clears the overgrowth, and admits strong light, 

 the plant speedily deteriorates. Kerner, in his Natural 

 History of Plants, draws particular attention to the 

 flora of the broad ridges and terraces of the rocky shores 

 of the Mediterranean, where the plants are exposed to 

 full light during the whole of their vegetative period. 

 They are " shrouded in dull grey, clothed in silk or wool, 



