772 THE SUBDIVISIONS OF THE VEGETABLE KINGDOM. 



in the form of switch-shrubs, and several Asclepiadacese, e.g. the species o^ tl^^ gf^^« 

 BUvdia, have cactiform steins. In these the assimilation of carbon is effected by 

 the green cortical tissue. The Orobanchace^ are parasites destitute of chlorophyU 

 (see vol i p 183). Amongst Convolvulaceae, and more especially amongst Scrophu- 

 lariace^, there are many species which live as parasites and saprophytes, and are 

 partially deficient in chlorophyll (see vol. i. pp. 171-183). An account has already 





Fig 434 — AcantliacedtJ 

 Acanthv^ nioUis on the coast of Dalniatia. 



iSfe^ 



been given of the way in which the Lentihulariacese, e.g. the species belonging to 

 the genera Utricularia and Pinguicula, derive a portion of their food from the 

 bodies of insects which are caught by them (see vol. i. pp. 120, 140). In Gentianaceae, 

 Oleacese, Apocynacese, Asclepiadacese, Convolvulacese, and many Boraginacese and 

 SolanacesB the corolla is actinomorphic. The Labiatse, Scrophulariacese, Verbenacese, 

 Acanthacese, Lentihulariacese, and some genera of Boraginacese and Solanacese bear 

 distinctly zygomorphic flowers. In the Ash genus (Fraxinus), which belongs to the 

 family of Oleacese, the corolla is often entirely suppressed. Most Labiatse have four 



