ADVENTITIOUS ASSOCIATIONS. 153 
berry (Rubus fruticosus and its allies). (3.) Those of Australian 
shrubs—e.g., the hakea (Hakea acicularis), various Australian heaths 
(three species of Epacris). (4.) Those of herbs and grasses—e.g., 
the milk-thistle (Silybum Marianum), the white clover (7vifoliwm 
repens), the horned poppy (Glaucium flavum), the foxglove (Digitalis 
purpurea), the giant fescue (Festuca arundinacea), and the marram 
(Psamma arenaria)—this also frequently an artificial association. 
(5.) Those of inland waters—e.g., the watercress (Radicula Nasturtium 
aquaticum), the cursed crowsfoot (Ranunculus sceleratus), the Canadian 
water-weed (Elodea canadensis), the yellow monkey-flower (Mimulus 
luteus), and the water-buttercup (Ranunculus aquatilis). 
A few of the above associations need some further consideration. 
In certain places where the manuka-association has been burnt out 
on the gumlands of Auckland the Australian Hakea acicularis now 
forms pure colonies, notwithstanding it had to compete with the 
manuka itselfi—a most aggressive plant. The exact circumstances of 
this case demand careful inquiry. One point of interest is that bio- 
logical literature appears to accept as a fact that Australian plants 
cannot become naturalized to any extent in New Zealand. Certainly 
there are at most but 20 purely Australian species amongst the 
naturalized plants; but of these one-half are firmly established as 
colonies amongst the indigenous vegetation, even if in some cases such 
establishment is restricted at present to only one or two localities. 
Nor does the fact of a species being so limited in its distribution, 
or of its being exceedingly rare, prove that it is incapable of becoming 
really aggressive. Thus a plant may remain quite isolated for years 
and be apparently incapable of spreading, but an unlooked-for change 
of conditions may give it just what it requires. Dr. Truby King, 
some years ago, pointed out to the author a most interesting case. At 
Waitati, near Dunedin, on the land belonging to the Mental Hospital, 
stands a fine example of a kind of stringy-bark (Eucalyptus nume- 
rosa) more than fifty-eight years of age. Originally the vegetation of 
the area was mixed taxad forest, but this has been replaced by a close 
growth of manuka thicket. Some eighteen years ago this thicket was 
burned in the neighbourhood of the tree, and a young forest of gums 
several acres in extent has sprung up, the new ground and the potash 
from the fire bemg eminently suitable for the germination of the 
gum-tree seeds. In 1910 the gum-saplings grew extremely closely 
(fig. 89). Their height was from 40 to 50 feet. Some were half a foot 
