THE LEAF 
5 ° 
or petiole , and the blade or lainina ; attached to the base are 
often found a pair of stipules (usually blade-like outgrowths), 
one on either side, e.g. in the rose. 
Even on the same plant the form is not always the same ; 
polymorphism (p. 29), here often called heterophylly , is fre- 
quent. Often, as in Hedera and other climbers, water- 
plants, epiphytes, insectivorous plants, Dischidia, &c. (see 
Chap. III. and Pt. II.) it bears some relation to differences 
in function, but in other cases, e.g. Capsella, Liriodendron, 
Bryophyllum, we cannot at present explain it. 
Many peculiarities of leaf-form are bound up with the 
necessity for reduction of transpiration in certain climates, 
soils, or positions (see Chap. III., Xerophytes, Epiphytes, 
Shore-plants, Alpine plants) ; others with the acquisition 
by the plant of a climbing habit or a water-habit. Others 
occur in Insectivorous and Myrmecophilous plants, &c. 
Others are connected with the storage of reserves for hiber- 
nation or for vegetative reproduction, as in bulbs, &c., or 
with the protection of delicate parts as in winter buds, where 
the scales so commonly seen on the outside are leaves which 
have abandoned their normal functions and normal structure 
to take over the function of protection and with it a suitable 
structure. Scale-leaves occur in other positions and may be 
mere functionless relics or vestigial organs (p. 31), e.g. on 
many plants whose stems have taken over the usual leaf- 
functions, or in saprophytes or parasites where the changed 
mode of nutrition deprives the leaves of their value as organs 
of assimilation. 
The acuminate apex so common in tropical leaves is 
apparently an arrangement for rapid drying \cf. Ficus, and 
Chap. III.). In many plants, e.g. Rutaceae, Guttiferae, the 
leaves show pellucid dots when held up to the light ; these 
are oil cavities in the tissue. Curious and often unexplained 
features are the holes in the leaves of Monstera and Aponoge- 
ton, the pockets of Xanthosoma, the blades in Codiaeum, 
the grooved petioles of Fraxinus. 
Like the leaf itself the stipules also show great variety 
of form ; *in Lathy rus Aphaca they do the assimilating work, 
whilst the rest of the leaf is transformed into a tendril, 
and in Azara, Viola, Rubiaceae, &c. they do a great deal of 
assimilation. Or they may be scaly and aid in bud-protec- 
