80 A TEXTBOOK OF GENERAL BOTANY 
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Hooks. Some climbing plants have leaves with hooks which | 
enable them to hold on to other objects. Excellent examples 
are afforded by the climbing palms known as rattans (Fig. 489). 
These have large, feathery leaves with numerous curved, sharp, 
pointed hooks (Fig. 70) which catch on to objects with which 
they come in contact and thus support the stem and enable the 
plant to climb. Some climbing plants have stipular hooks which 
serve the same purpose. 
Floats. The floating 
plants develop very 
loose tissue with large 
air spaces (Fig. 71). 
Such tissue enables the 
plant to float on the 
surface of the water. 
The bulbous petiole of 
the water hyacinth is 
occupied largely by air 
spaces and presents a 
striking case of the de- 
velopment of floating 
tissue (Figs. 71, 72). 
: Similar tissue is found 
Fie. 68. Tip of stalk of sugar cane. (x sy in the blades of float- 
ing leaves (Fig. 5). 
The air spaces in plants serve for aération, that is, the ex- 
change of gases between the cells and the external atmosphere. 
Frequently the large air spaces in aquatic plants are primarily 
useful for this function, as is particularly true of the large air 
tubes which traverse the petioles of water lilies (Fig. 40) and 
by which the oxygen set free in the leaves by photosynthesis is 
allowed to diffuse to the roots. 
Absorption (root function). One of the chief functions of roots 
is to absorb water as well as compounds of nitrogen and other 
elements which plants usually obtain from the soil. Most leaves 
are incapable, under ordinary circumstances, of absorbing these 

