II] SHEATHS AND STIPULES 19 



and to obtain light and air more advantageously, and so 

 favour the leaf. 



The insertion of the petiole on the stem may be 

 narrow and small, or broad and large ; and when the leaf 

 falls the scar or cicatrix left on the stem is often very 

 characteristic in shape, e.g. Magnolia, Ash, Horse-chest- 

 nut, Hickory, Platanus, Walnut, details of use in deter- 

 mining the species of deciduous trees in the winter, as 

 explained in Vol. I. p. 118. 



In many Rammculacea?, UmbelliferaB, &c., the base of 

 the petiole is so dilated that it surrounds the stem more 

 or less completely with a membranous sheath, which often 

 carries the point of divergence of the petiole some distance 

 above the nearly circular insertion. Such a sheath is still 

 more conspicuous in the case of Grasses and Sedges, where 

 it often surrounds the stem like a long rolled-up cylinder, 

 and passes above either into a very short true petiole 

 (Bambusa) or, more usually, directly into the lamina and 

 constitutes all that there is to represent the petiole. 



The leaves of Mahonia are also sheathing, the base 

 being somewhat wrapped as it were round the stem. 



In a large number of cases, again, the base of the 

 petiole has two lateral appendages flanking it, and so far 

 recognisable as distinct organs, more or less leaf- like in 

 character, that they must be distinguished by the name 

 of stipules. These stipules vary much in size, shape, 

 position, and degree of segmentation; and as they often 

 serve special purposes, such as protection of tender young 

 organs, and aid in the diagnosis of whole families of 

 dicotyledonous plants, they have received considerable 

 attention from botanists. 



Stipules may be free, as in the Elm, Quince, &c., or 

 adnate to the petiole, as in the Clover, Rose, &c., or they 

 may be so completely united to the latter and to each 



22 



