DURATION AND RETENTION UPON THE PLANT 175 



through the union of the adjacent stipules, of four. In such cases, the 

 leaves which are stipules can be distinguished from the others by their 

 failure to develop axillary buds. The stipule is frequently transformed 

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

 a tendril (Fig. 565). 



A secondary stipule, borne at the base of one of the divisions of a 

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



The Petiole. — Leaves possessing the Petiole; are called Petioled 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 Clas jji ng 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 Margined or 

 Winded. 



When the margins of the petiole are less pronounced, but yet present 

 and elevated, so as to form a groove upon its upper surface, the petiole 

 is called Channelled. 



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

 form in transverse section, its relative 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 petiole to the leaf-blade is always really 

 marginal, though by the cohesion of basal lobes (Fig. 473, a) it is often 

 apparently intra-marginal or even central. Basal lobes may, upon the 

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

 be produced by the gradual differentiation of petiole into blade. 



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

 part of the petiole, and their appearance is characteristic and of diag- 

 nostic value, as in distinguishing the species of Prunus and Casfiia. 



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

 and its retention upon the plant have to do in part with the nature of 

 the petiole, it may be here considered. Leaves are Annual,, and the 

 plants producing them deciduous , when their duration is through a 

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

 and active condition into the succeeding season. Evergreen leaves 



