772 



THE SUBDIVISIONS OF THE VEGETABLE KINGDOM. 



in the form of switch-shrubs, and several Asclepiadaceae, e.g. the species of the genus 

 Sta^pelia, have cactiform stems. In these the assimilation of carbon is effected by 

 the gre^-^n cortical tissue. The Orobanchacese are parasites destitute of chlorophyll 

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

 lariacese, 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 



'''^. .'' r-C^ 



Fig. 434. — AcanthacesB. 

 Acanthus mollis on the coast of Dalmatia. 



>Vi ''^^ -^/^ 



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

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

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

 Oleacese, Apocynacese, Asclepiadacese, Convolvulacese, and many Boraginaceae and 

 Solanacese the corolla is actinomorphic. The Labiatae, Scrophulariacese, Verbenacese, 

 Acanthaceae, Lentibulariaceffi, and some genera of Boraginacese and Solanaceae 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 



