THE DICOTYLEDONES 1693 



or may be more or less sunk into the floral axis. There are numerous ovules 

 arranged on parietal placentas. These ovules possess two integuments, 

 except in one family, the Loasaceae, in which there is only one. The seeds 

 usually have endosperm. The plants are either woody or herbaceous and 

 possess either opposite or alternate leaves often provided with stipules. 



As originally conceived by Engler this order was a large one embracing 

 several groups, the relationships of which are far from obvious. Hutchinson 

 has therefore divided the Parietales into a number of separate orders and 

 redistributed these in his system of classification. Thus he includes the 

 families Cistaceae, Bixaceae, Frankeniaceae and Flacourtiaceae together 

 in the order Bixales. The Tamaricaceae are placed alone in the Tamaricales, 

 the Violaceae in the Violales, the Loasaceae in the Loasales and the Passi- 

 floraceae in the Passiflorales. Finally two families are included in other 

 Englerian orders, the Caricaceae in the Cucurbitales and the Elatinaceae 

 in the Centrospermae (Caryophyllales). We need not discuss here the 

 details upon which these views are based, since only a single family, the 

 Violaceae, will be considered in detail in this book. We should however 

 note some of the characteristics of certain of the more important families 

 which are embraced in the Parietales as originallv defined. 



The Cistaceae are small herbaceous or shrubby plants, some of which 

 are strongly calcicole while others are calcifuge. Among the more 

 important members are Cistiis (Fig. 1554) and Helianthemum (Rock Rose), 

 one species of the latter occurring commonly in this country while three 



Fig. 1554. — Cistus ladanijerus. The flowers are two 

 to three inches in diameter. 



more are very local. It is interesting to note that a mycorrhizal fungus 

 has been demonstrated in the roots of a number of species, which also 

 appears to be present in the gelatinous wall of the seed. These plants 

 inhabit dry, sunny situations and it may be that the mycelium assists in 

 the early development of the seedling. 



