THE LEAVES OF PLANTS. 



101 



In other instances the leaflets are not arranged in a pinnate manner, but form o 

 kind of tuft, as in Figs. 175 and 176 ; but even in Such cases there is no lifnculty in 

 tracing an analogy between them and the whorled form of leaves shown in Fig. 179. 



"We intimated at the head of this section that there is another form of compound 

 leaf besides that now described, and it is one which is based upon distinct anatomical 

 characters. It is such leaves as are connected with 

 the petiole by means of an articulation or aa im- 

 moveable joint. If the leaf of an apple or -an oak 

 tree be examined, it will be seen that the midrib 

 passes uninterruptedly down into the petiole ; but 

 the leaf of an orange presents a transverse line with 

 a slight swelling on either side of it (Fig. 181 a), and 

 at this point the blade of the leaf may be somewhat 

 readily broken from the petiole. There is no arrest of 

 circulation at this place, although the separation is 

 easily effected, for the vessels pass uninterruptedly 

 from the petiole to the midrib. It is thus not easy 

 to show how or why such an anatomical peculiarity 

 should exist ; for the common opinion, that it is the 

 terminal leaflet of a compound leaf with the lateral 

 leaflets undeveloped, does not much help us. It is 

 also found in the common berberry (Herberts vul- 

 garis), and in a few other plants. 



We have already stated that a leaf without a 

 petiole is termed sessile, or sitting, but when it en- 

 tirely surrounds the stem it fs known as perfoliate (Fig. 182), and when it runs down 

 the stem, as in certain 

 thistles, it is called decur- 

 rent (Fig. 183). 



The petiole, or foot- 

 stalk of the leaf, is the 

 assemblage of the veins 

 of the leaf which con- 

 ducts the juices to and 

 from the stem. As it 

 contains all the vessels 

 of the leaf it must pos- 

 sess two sets of vessels, one devoted to the conveyance of fluids to, and the other 

 from, the leaf. There are also spiral vessels and so much cellular tissue and 

 cuticle as may connect and inclose the vessels in the most compact forms. The figure 

 of the petiole is rounded ; but in many instances the upper surface has a channel, and 

 thence is called gutter-shaped. In other cases it is perfectly flat, or has processes on its 

 sides which give it the appearance of winged ; or it is rigid, twisted, or hooked. The 

 grasses and the Ranunculacese have a sheathing petiole, or one which passes down the 

 stem, and is so large as nearly to embrace it. It has at its point of connexion with 

 the blade a little organ found universally in grasses, called the ligulu (Fig. 184 a). The 

 petioles of the leaflets of a compound leaf are termed petiolules. 



The distal extremity of the petiole is the part first formed in the bud ; fcnd when at 



Fig 181. The compound leaf of the 

 orange, with the articulation re- 

 presented at a. 



Fig. 182. Aper- 

 foliate leaf. 



Fig. 183. A decurrent leaf, with the midrib adhe- 

 rent to the sides of the stem. 



