110 MORPHOLOGY OF LEAVES. 



Baptisia perfoliata, shown in Fig. 214, are not simple blades, 

 but each a pair of stipules, with or without a terminal leaflet, all 

 completely confluent into one body. The related species of the 

 genus have trifoliolate leaves and foliaceous stipules ; hence these 

 simple leaves without stipules are best explained in this way. 



217. Phyllodia, or Petioles serving for Blade. Sometimes the 

 petiole develops foliaceous margins, or wings, as in the Bitter 

 Orange and in Rhus copallina. These are efficient as foliage in 

 proportion to their size. These are not to be confounded with the 

 case in which a petiole specially develops as a blade-like organ, 

 which usurps the office of foliage. A petiole-blade of this kind 

 is named a PHYLLODIUM. Occurring only in Exogens, phyllodia 

 are generally distinguished from true blades by the parallel 

 venation, and always by their normally vertical dilatation ; i. e. 

 they, without a twist, present their edges instead of their faces to 

 the earth and sky. The common and most familiar phyllodia are 

 those of Acacias in Australia (Fig. 223, 224), where they form 

 the adult foliage of over 270 out of less than 300 species. The 

 true lamina of these is bipinnate. It appears on seedlings, and 

 occasionally on later growths. Several South American species of 

 Oxalis produce phyllodia. So likewise do our tubular or trumpet- 

 leaved species of Sarracenia in that portion of the foliage which 

 develops the pitcher imperfectly, or not at all. Indeed, all 

 Sarracenia-leaves are phyllodia with the back in most of them 

 hollowed out into a tube or pitcher ; and the terminal hood 

 answers to the blade. 



3. LEAVES SERVING SPECIAL OFFICES. 



218. Leaves may serve at the same time both their ordinary 

 and some special use, or even more than one special use. For 

 example, in Nepenthes (Fig. 222) there is a well-developed 

 blade, usually sessile, which serves for foliage, a prolongation of 

 its tip into a tendril, which serves for climbing, then an extraor- 

 dinary dilatation and hollowing of the apex of this into a pitcher 

 for a very special use, and a peculiar development of the apex 

 of this into a lid, closing the orifice during growth. Among the 

 special purposes which leaves subserve, and the study of which 

 connects singularities of morphology with teleology, the most 

 remarkable is that of 



219. Leaves specialised for the Utilization of Animal Matter. 

 This occurs in leaves which also assimilate, or do the ordinary 

 work of vegetation ; and the special function is usually taken up 

 by some particular portion of the organ. The details of this 



