GENERAL MORPHOLOGY OP THE PLANT 59 



the Grasses it surrounds the stem in the form of a sheath {fig. 79)j 

 and where the blade and petiole join there is found a membra- 

 nous appendage, Ij'ing parallel to the blade. To this the name of 

 ligule has been given. It is either entire or incised in various 

 ways. In the Aspen {Fopulus tremula), the petiole is flattened 

 in a line at right angles to the blade, and is thus one of the 

 causes of the peculiar mobility of such leaves ; in other plants 

 it is flattened in a horizontal direction. In aquatic plants the 

 petiole is frequently more or less dilated from the presence of a 

 number of air cavities, as in Pontederia ; such petioles, by 

 diminishing the specific gravity of the plants in which they are 

 found, enable them to float readily in the water. In many plants 



Fig. 93 



Fig. 94. 



Fig. 93. Imparipiunate or equally pinnate leaf 

 of Rohinia, -with spiny stipules.^ — Pig. 94. 

 Paripinnate leaf. 



the petiole is winged like the leaf-blade, though generally not to 

 the same extent. Instances are found in the Orange (fig. 92) 

 and the Venus's Fly-trap {fig. 91). 



In some eases the epipodium is not developed, or falls off at 

 a very early age. The petiole is then usually very strongly 

 winged, and takes on the ordinary function of the blade. This 

 is seen in some of the Leguminoss ; Laihyrus NissoUa of the 

 British flora is an instance. It is much more conspicuous in 

 many of the Australian Acacias ; some of their leaves show the 

 ordinary pinnate form ; others are similar with the petiole 

 winged, and others show the winged petiole only, the epipodium 

 having disappeared. Such a winged petiole is called a phyllode, 

 and can be distinguished from a leaf-blade by the fact that its 



