PLASTICITY OF THE MANUKA. 21 
Many species are readily changed in form according to change 
in the conditions to which they are exposed; im other words, they 
are plastic. The manuka (Leptospermum scoparvwm) when not ex- 
posed to excess of wind, and growing in fairly good soil in an area 
where the rainfall is abundant, may be a small tree, 30 feet high, with 
a trunk | to 2 feet in diameter. When growing in its usual station 
in poor soil as a member of the manuka-thicket plant-formation, 
it is a rather tall shrub with its branches more or less erect and 
not spreading. On a sour, wet, cold soil m the mountains it 
creeps over the ground with its slender stems, which root into the 
soil; indeed, it may form a veritable turf .(fig. 11). Finally, it 
may bloom as a seedling when only an inch or two high. Thus 
one and the same plant, according to circumstances, can assume 
four growth-forms which are absolutely distinct from one another 
both in appearance and in physiological requirements. Were the 
case of the manuka a solitary one it could not be taken as an 
argument in support of any theory; but, as will be seen, such 
extreme plasticity is so common amongst New Zealand plants as, 
after a time, to occasion no surprise; and the question arises, Is 
there no true fixity, and are the forms of all the plants only main- 
tained so long as the environment remains more or less constant ? 
The change in form of a growth-form or of an organ is the out- 
ward mark of a physiological response of a plant to some stimulus, 
which may be in the nature of a greater or lesser amount or intensity 
of light, heat, or moisture reaching it. More or less of a certain salt 
in the soil, the “* sting” of an insect, or the presence of a fungus may 
also act as stimuli. 
How far such action of a stimulus to which a plant has not been 
accustomed, carried on for a long time, can make itself felt so as to 
produce a more or less permanent change of form is a matter in great 
dispute. Be that as it may, the New Zealand flora offers no small 
amount of evidence that such changes as the environment can bring 
about may become very deep-seated. Further than this it is not 
safe to go in an elementary book, where speculation should take a 
quite minor place. 
Heat, light, moisture, the soil—this a remarkable complex ; the 
animals with which the plant comes in contact, as also the plants 
with which it is associated—all these are factors operating upon the 
plant which may be either beneficial or harmless, otherwise it must 
