DURATIOS AM) RI'JTKNTION UPOX TIIK I'LAST 175 



tlirouiili the union of the adjacent stipules, of four. In sncli cases, the 

 k'a\cs \vhich are sti])ulcs can be distinguished from the others by their 

 faihu-o to develop axiHary buds. The stipule is frequently transformed 

 into one or more bristles, or even strong spines, and occasionally into 

 a tendril (Fig. o()r)). 



A secondary stipule, borne at the base of one of the (li\isions of a 

 compound leaf, is called a StipeUa (Fig. 475, a). 



The Petiole. — Leaves possessing the Petiole; are called I'etioled those 

 wanting it are called Sessile (Fig. 478). Occasionally the petiole is 

 present, but adnate to the stem of the plant, thus appearing wanting. 

 In other cases, while quite free from the plant-stem, more or less of 

 the base of the petiole will clasp it. Such a Clasping petiole must not 

 be mistaken for a leaf-sheath, which, as we have seen, is not a true 

 petiole at all, but the development of a different part of the primary 

 leaf. 



When the margins of the petiole throughout are herbaceous and in 

 continuation with the blade, the petiole is said to be INIargined or 

 Winged. 



When the margins of the ])etiole are less ])r()n()unced, ])ut yet present 

 anfl elevated, so as to form a groove upon its upper siu'face, the petiole 

 is called Channelled. 



Other characters of the petiole, such as its triangular or semicircular 

 form in transverse section, its relati\-e stoutness, and the character of 

 its surface, need not be specially considered. Certain special modifica- 

 tions in the function of the petiole will be considered under modified 

 leaves. 



The attachment of the i)eti()le to the leaf-blade is always really 

 marginal, though by the cohesion of liasal lobes (Fig. 47i), a) it is often 

 apparently intra-marginal or even central. Basal lobes may, ui)on the 

 other hand, be adnate along the petiole, or the same appearance may 

 be produced })y the gradual difi'erentiation of petiole into blade. 



Petiolar Glands. — (ilands of various forms often appear upon some 

 part of the jx'tiole, and their appearance is characteristic and of diag- 

 nostic value, as in distinguishing the species of Pnniu.s and Cassid. 



Duration and Retention upon the Plant. — As the duration of the leaf 

 and its retention upon the i)lant ha\e to do in part with the nature of 

 the petiole, it may be here considered. Lca\es are Annual, and the 

 ])lants producing them deciduous, when their duration is tluough a 

 single season only, and Fvergreen, when they remain in their normal 

 and active condition into the succeeding season. Evergreen leaves 



