Where a valley constitutes the boundary line betwen neighbouring districts, it commonly happens that the alpine and subalpine vegetation on either side halts abruptly, so that none of the characteristic subalpine plants of one district is to be found in the other; while the lowland flora comprises forest on the side of greater rainfall, and grassland on the side of lesser rainfall. This is conspicuously the case in the lower Wairau Valley, which separates the S.N. and N.E. districts, and again in the Waihopai Valley between the N.E. and N.W. districts, though in the latter case much of the forest has been removed by fire.

Apart from widely distributed species common to the whole province, few of the characteristic plants of any one district will be found to cross these valleys and invade the neighbouring territory. The few exceptions as noted by the writer are as follows:—