360 PLANTS OP NEW ZEALAND 



forming a huge disk that prevents the mangrove from being 

 upset. The tree is further propped up by a number of stilt 

 roots. These, however, are not so large or so long as in the 

 typical mangrove {Uhizopliora mangle) of the tropics. 



The trees, thus buttressed, stayed, and fixed by their varied 

 roots, stand firm in the highest tide and the strongest flood, 

 and gather about them material for a rich soil, which 

 gradually becomes suitable for cultivation. Then they step 

 farther out into the water and begin their work again. So far 

 as can be roughly estimated, the mud-flats are thus raised by 

 the action of the mangroves, on an average, a little over an 

 inch each year. This rate of speed is probably only that of the 

 present time, as it must have been much slower in past years, 

 before the bush was so extensively cut. Since the clearing of 

 the land, slips have become much more frequent, and hence 

 the rate of deposition is now faster than before. 



The Germination of Avicennia. 



It is obvious that, if the seed of the mangrove fell 

 undeveloped into the mud below the tree, it would be liable to 

 be carried off by the rising tide. To avoid this fate, the 

 embryo in many mangroves passes through no resting stage, 

 but continues to grow until its weight breaks it away from 

 the seed-case and from the tree. Thus the young plant of 

 Bhizophora mucronata sometimes attains the length of two 

 feet, before it falls from the parent tree. It then drops 

 vertically downwards, and its weight and club-shaped form 

 cause it to penetrate for some distance into the mud, where it 

 remains standing in an upright position. According to 

 Schimper, within the short space of a few hours, it produces 

 roots which fix it firmly in position. Mediaeval travellers, not 

 content with this marvellous series of adjustments to environ- 

 ment, spoiled the story by stating that the mangrove trees 



