THE LEAF 143 
air, and intercept such rays of broken sunlight as have 
filtered down through the taller plants above. 
Variety of leaf form is a striking characteristic of 
water plants. In the leaf of the water-lily, for 
instanee, which spreads itself on the surface of the 
water where there is not likely to be interference by 
other plants, the blade is broad and expanded, indeed 
almost round in form. Where, on the other hand, the 
leaves of a water plant are submerged, they are 
divided into many segments, that they may wave to 
and fro and secure what little carbon dioxide there 
is dissolved in the liquid around them. This point is 
well illustrated by comparison of the aérial and sub- 
merged leaves of the watercress. The former are but 
little divided, whereas the latter are cut up into 
numerous segments. 
Division of the leaf blade often serves other 
purposes. <A leaf divided into small segments is not 
so likely to be torn by wind or water as one in which 
the blade is broad and presents a large and unyielding 
surface. This, in part, accounts for the division of 
the submerged leaves of the watercress. 
The advantage of a divided blade is further 
strikingly illustrated by a comparison of the effect 
of wind in the huge undivided leaves of the banana 
tree and on the leaf of the kowhai with its many 
small leaflets. In the Albert Park at Auckland the two 
plants are growing in close proximity, and, while the 
former is ripped and torn, the latter remains 
practically unharmed. 
The apex of a leaf may be acute, as in the willow 
and tawa, obtuse as in the rewarewa (Knightia) and 
pungent, as in the totara and tea-tree, where it has a 
hard, sharp tip. Leaves which, like those of the wall- 
flower, the tea-tree and miro, have no petiole, are 
sessile, while all others are petiolate. 
