ON LEAVES. 83 
of the leaf towards the summit, all being parallel to each 
other. 
The petiole, sometimes long, sometimes short, and sometimes 
absent altogether, is often cylindrical, sometimes arched and 
inflated as in the Water Caltraps (Zrapa natans), or compressed, 
as in the Birch-tree (Carpinus betulus), and many poplars, in 
which the surface is large, in place of being a continuation of the . 
blade, it abuts upon it at right angles. In such cases the petiole 
gives little support to the leaf, but presents its two largest sides 
to the wind, which cause it to oscillate and tremble, producing the 
rushing sound which distinguishes the Aspen-tree (Populus 
tremula). 
Tn some cases the petiole fails; the blade even may be defective ; . 
the leaf is then reduced to its petiole. But in such cases, in 
obedience to the laws of compensation, which intervene when any 
organ p bortive, the neichl i 
, g g parts take a greater develop- 
ment; the petiole is enlarged and assumes a ribbon-like form, a 
sort of blade which was long taken for a leaf. It is distinguished 
by the position of its veins, and also by the fact that in place of 
being compressed in such a manner as to present the usual upper 
and under surface, it is set edgeways, and its two faces are lateral. 
This form of petiole is termed a Phyllodium, and is applied to so 
called leafless plants, where the petiole performs the functions of 
one. The Acacia heterophylla, Fig. 110, is full of instruction in 
this respect. We find in it all the intermediate steps between a 
perfect compound leaf and a phyllode. The petiole is there seen to 
flatten and enlarge in exact proportion as the leaf decreases ; they 
bear leaflets at the earliest stage of their development, and have 
parallel veins, although occurring in exogenous plants. This trans- 
formation’of the petiole, which is frequent in the Acacias of Australia, 
occurs also in many other plants, as in Dionea muscipula, Venus — 
Fly-trap, in which the petioles extend laterally, and resemble true 
leaves; in the tendrils of the pea, and in some others belonging to 
the leguminous plants, as well as among the umbelliferous plants 
and the ranunculacew, the petiole grows longitudinally. 
Leaves transform themselves into other organs with wonderful 
facility. It is, in fact, by modifications of the leaves that nature 
produces many essential organs in the life of plants. They are 
