54 NEW ZEALAND PLANTS. 



In 1886 the eruption of Tarawera led to the burying of large areas 

 of this plant society by volcanic ash. So thickly did this fall that in 

 some places an actual new land-surface was formed for repopulation. 

 This was of great interest, since opportunities for observing the settle- 

 ment of a large area of virgin soil under natural conditions are rarely 

 afforded ; and in this case there is a clue to what may have taken 

 place long ago in the evolution of the plant-covering of the adjacent 

 country. 



Where the heath was but thinly covered, it has reappeared almost 

 in its original form ; but where the covering was many feet in depth 

 there is quite a different story. Very shortly after the eruption heavy 

 rain occurred, and the comparatively loose soil was cut into innumer- 

 able deep but narrow gullies, with many lateral ones opening into them. 

 The sharp ridges between these gullies are bare, but on their sides 

 wave masses of toetoe grass (Arundo conspicua), a plant not very 

 abundant in the adjacent heath. The "seeds' of this grass would, 

 of course, be brought by the wind. Another common member of the 

 new r society is the tutu (Coriaria rusci folia), its ' seeds," of course, 

 having been brought from the plants of the adjacent heath by birds. 



ADAPTATIONS OF THE HEATH PLANTS. 



AVithout water, plants cannot exist. It may therefore be expected 

 they have developed many special contrivances to insure the necessary 

 supply. Especially important are those connected with water storage 

 or saving, where there is danger of drought. The clay hills on which 

 the northern heath nourishes, although soaked with water in the 

 winter, become exceedingly dry in the summer, and had the plants 

 no provision for husbanding their water they would die for lack of 

 moisture. 



The leaves of plants serve various purposes. On their under- 

 surfaces usually are many most minute openings, too small by far 

 to be seen with the naked eye, which afford communication between 

 the interior of the leaf and the atmosphere. Through these openings 

 a constant stream of water-vapour issues, and through them, too, 

 a perpetual current of air enters. From the air the plant gets its 

 oxygen for breathing, and its carbonic acid for food purposes, while 

 the water passing away makes room within the plant for a fresh supply 

 charged with nutritive matter drawn from the soil by the roots. If, 



