DEVELOPMENT OF PLANTS 



437 



types through the same series of variations, as noted in the 

 Choripetalae. The Sympetalae are the most recently evolved 

 of the Angiospermae, as is apparent from their uniformity of 

 structure and they appear to be adapted to temperate and north- 

 ern conditions where they have preempted the open country and 

 flourished exceedingly, forming the most characteristic features 

 of the herbaceous flora. Aquatics are of rare occurrence and 

 but few tree forms appear ; notably the ash, persimmon, catalpa, 

 paulownia, etc. Heath-like shrubs, however, are widely distrib- 

 uted in northern regions. The invasion of the tropics with its 

 favorable conditions has led to an enormous increase in some of 

 the groups and the development also of a great variety of woody 

 plants, as trees and climbers. 



150. Ericales, the Heath Order. — This order is the simplest of 

 the Sympetalae and includes a great variety of plants that are 

 largely northern in their distribution. Many are cultivated, as 

 the azaleas, rhododendrons and laurels (Kalmia), and others are 



Fig. 326. A simple form of the Ericales : A, the shin leaf (Pyrola) with 

 flower just opened. B, flower with part of perianth removed to show the 

 retention of the anthers by the petals. C, later stage of flowering. The 

 anthers have been released by the spreading of the petals and the flower 

 stalk has inclined so that the spores sprinkle down on the stigma. D, 

 flower in section, showing the relation of parts in autogamy. — After 

 Kerner. 



