DEVELOPMENT OF PLANTS 473 



arctic regions where they constitute the conspicuous features of 

 the tundras, bogs, heaths and moors. These plants are often 

 characterized by their thick leathery, evergreen leaves of pro- 

 nounced xerophytic character and their common distribution 

 in wet moors and bogs presents a most puzzling problem in the 

 association of plants. Many of the heaths have been cultivated 

 from very early times and they have lent themselves so readily 

 to crossing that perhaps no order furnishes so many forms for 

 decorative and landscape effects. Several yield valuable fruits, 

 as the blueberries, huckleberries and cranberries. 



151. Orders of a Higher Type than the Ericales. — The prim- 

 rose order (Primulales) forms a very natural transition from the 

 Ericales. As the next step in advance, we note the slight associ- 

 ation of the stamens and the corolla (Fig. 328, A) and their 

 frequent reduction in number. The occurrence of staminodia, 

 page 421, in certain forms marks the first transition to the 

 reduction of the number of stamens. The pistil is generally 

 composed of five carpels forming a capsule without partitions 

 (unilocular) and containing a central placenta, bearing numerous 

 seeds, that is attached only to the base of the ovary (Fig. 328, A). 

 This is the so-called free central placenta and only occurs else- 

 where in the small bladderwort family of aquatics. This order 

 includes the loosestrife (Lysimachia and Steironema), star flower 

 (Trientalis), shooting star {Dodecatheon) and cultivated forms of 

 the cyclamen and primroses. 



Proceeding to the gentian order (Gentianales), it will be 

 noted that the flowers are variable and not as clearly character- 

 ized, the corolla being sometimes wanting or polypetalous. The 

 reduction of the stamens to a single whorl and the pistils usually 

 to two in number indicates points of advance that are to be 

 associated with the twisting of the petals in the bud and the 

 opposite arrangement of the leaves as distinguishing features 

 of the order (Fig. 328, B-D). Note should be made of the fact 

 that the two pistils are often quite distinct, doubtless a survival 

 of a more primitive condition. The Gentianales include the ash 

 {Fraxinus), lilac {Syringa), Forsythia, olive (Olea), valued for 

 fruit and oil, fringe tree (Chionanthus) , privet {Ligustrum), jas- 



