THE LEAF 133 
on the inside of the stem are thinner than the walls of. 
those on the outside the diffusion is more rapid there.. 
The greater swelling of the internal cells thus pro- 
duced causes the strip to bend with these inner cells: 
on the outside. If the piece of stem be left lone 
enough in the aleohol it will form a coil. Now place: 
the same strip in strong brine. The alcohol and water 
will by osmosis pass out of the cells more rapidly than 
the brine passes in. The eells will, therefore, lose 
much of their liquid contents, and the strips become 
so limp that, when suspended over the finger, they 
hang vertically much as is the case with the flaccid 
cuard cells (Fig, 98). | 
All ordinary plants cheek undue transpiration by 
closing their stomata, but there are many plants, 
which, growing in situations where moisture is scanty 
and its conservation important, have special peculiari- 
ties of structure that adapt them to their surroundings 
by guarding against loss of water. Such plants are 
ealled drought-forms or xerophytes (Gk. zeros dry and 
phyton a plant). The great number of drought forms 
found in New Zealand indicates that the climate of this 
country was onee much drier than it now is. The 
most obvious way of checking transpiration is_ to 
reduce the number of stomata; and this is one of the 
means employed in the karaka, laurel, and camellia, 
in which the stomata are practically all on the lower 
surface. 
A thickened epidermis, which assists some plants to 
conserve water, 1s also seen in the leaves of the kauri 
and totara. 
Leaf reduction (Fig. 94) is perhaps the commonest 
means of adaptation to dry conditions. In the rimu 
or red pine the leaves are reduced to mere scales 
running in rows along the pendent branches, while in 
the celery-leaved pines, the toa-toa and tanekaha, as 
well as in the New Zealand broom or carmichaelia, the 
