50 



MORPHOLOGY, OE COMPABATIYE ANATOMY, 



The stalk-like petiole (fig. 55), most common in Dicotyledons, always 

 has the base slightly widened out at its point of emergence from the 

 stem ; in the leaves of Palms, the Banana, Scitaminese, &c. the base is 

 expanded so as to embrace the stem, while in the Grasses the petiole is 

 entirely represented by a sheath (fig. 59). The green part of the leaves 

 of the Hyacinth and other bulbous plants is the blade, and will be found 

 continuous below with a colourless, fleshy, petiolar portion, forming one 

 of the "coats " or sheaths of the bulb (fig. 17, p. 25). 



The leaf may, however, be represented by one only of the regions. 

 It is very common to find leaves without distinct petioles, the blade 

 springing directly from the stem: such leaves are called sessile 

 (fig. 56). On the other hand, tbe petiolar region may exist with- 

 out the blade ; and among the cases of this sort a considerable 

 variety of conditions is met with. Petiolar structures, devoid of 

 laminae, and more or less reduced to scales or membranous sheaths, 

 are commonly found on subteranneous stem-structures, such as 

 bulbs, rhizomes, &c., whence we have denominated this part of 

 the stem the " leaf-scale region." Similar scales appear in place 

 of green leaves in the "true-leaf" region of various parasitic 

 plants, such as Orobanche, in which the leaves have no physio- 

 logical function to perform; and they recur periodically on the 

 stems of arborescent plants which form winter buds, in the shape 

 of bud-scales. In the true-leaf region the blade is either supported 

 on a stalk-like or sheathing petiole, or is sessile. The sessile con- 



Fig. 57. 



Fig. 58. 



Fig. 56. 



Fig. 56. A sessile leaf. Fig. 57. Phyllodium of an Acacia. 



Fig. 58. Two phyllodia of Ozali* latipes, one with a ternate Wade. 



dition is generally more common toward the upper part of stems 

 and shoots ; and in the bracts or leaves belonging to the inflores- 

 cence the petiolar region is comparatively seldom developed. The 



