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New Zealand Plants and their Story

The Mangrove

page 74

The Mangrove.

Let us leave the dunes, and, in imagination, sail up one of those wide estuaries in the west of the Auckland, Province—Hokianga or Kaipara Harbour, or one of the tidal rivers of the east—the Whangarei, for instance. If it is high tide, we shall see on either side of the stream a belt of close-growing, dull-coloured, small trees, rising out of the turbid water. These consist of the mangrove (Avicennia officinalis), and the sight is one almost unknown in any other land outside the tropics. It is, in fact, one of the natural wonders of New Zealand.

Now, quite undeservedly, the mangrove has got a bad reputation. A mangrove swamp is supposed to represent all that is most hideous on earth—alligators in crowds, a fearsome odour, crabs waiting to pick such of the victim's bones as are left by the alligators, malaria, and deadly microbes in vast abundance. Even in the tropics this picture has been shown to be absurd, and in New Zealand the mangrove belt is quite a pleasing feature of the northern rivers. It is also a most beneficial plant, as it materially assists in turning muddy useless shores into good dry land.

Moreover, the mangrove is one of the most noteworthy plants in nature. As our boat proceeds up the river the tide has turned, and the slimy flats, where the mangrove is rooted, come into view. There, projecting out of the mud, are thousands of upright bodies, 6 in. or so in length, looking much like stout asparagus-shoots. One might feel sure these were young mangroves. But they are nothing of the sort, strange as it may seem. They are roots, which, instead of passing downwards to anchor the trees, grow upwards into the air. On being examined, they are found to consist largely of a very porous tissue. Plants, like animals, cannot live without oxygen. They need to breathe just as much as we human beings do; without air they would die of suffocation. In the soft mud is little of the life-giving gas hence the necessity for the mangrove to obtain a supply for its ordinary roots. This it does with these erect organs, which are the veritable lungs of the tree. Of course, the aerial parts of the mangrove, like those of any other tree, procure oxygen by means of the small pores in the leaves and minute openings in their bark.

The mangrove, too, has another peculiarity of even greater interest than that just described. If a seed were to fall on the muddy floor of a tidal estuary, being washed hither and thither by the ebb and page 75
Fig. 33.—The Round-leaved Shrubby Groundsel (Senecio rotundifolius). Shore of Paterson Inlet, Stewart Island.Lands Department.] [Photo, L. Cockayne.

Fig. 33.—The Round-leaved Shrubby Groundsel (Senecio rotundifolius). Shore of Paterson Inlet, Stewart Island.
Lands Department.] [Photo, L. Cockayne.

page 76flow of the tide, it would have little chance of germinating Consequently, the embryos in the seeds of the mangrove develop considerably while still on the tree, emerging from the seed and producing rudiments of roots ready for rapid growth. When such young plants fall from the tree, the roots grow rapidly. They pass downwards and outwards from near the tip of the stem below the seed-leaves, and so anchor the plantlet firmly in the unstable ground. Nor is this all. The seed-leaves are fleshy and full of nourishment, and on this the young mangrove lives in part* until the time when, provided with foliage, it is in a position to manufacture for itself the sugary foods it requires from the atmosphere and the water. Surely none need cast contumely on such a plant as this!